Hello all,
I have come to the end of my patience and frustration and am hoping someone out there has had the same experience and can point me in the right direction with this issue.

I have a 2008 GMC Savana 1500 AWD with 145,000 miles. I just purchased it so I don't know the history but it is overall in really good looking shape so I had no reason to think it was abused.
Here are the highlights, I'll go into the story further below.

-Vibration started out of nowhere at 50 mph. Bad vibration, like unsafe.
Replaced both front hubs, control arms, tie rods, stabilizer bar links, upper and lower ball joints, aligned. They also added a bit of fluid to the transfer case.
This changed the vibration and made it a lot better. Now it only happens above 70 or so and is not as bad.

-Brought back in. Found the rear yoke was seized in the transfer case. Everything rotated ok but it wouldn't come out of the transfer case. Had a rebuilt transfer case put in, and used rear driveshaft as they had to cut it to remove it from the transfer case.
-I thought it was fixed, didn't notice the vibration but it was winter and I had studded tires on it so the road noise was a little loud. Come spring I put on the original tires. Vibration came back, or never went away. Feels the exact same to me but mechanic thinks it is different.
-Brought back in. Mechanic thought the tires were "feathered" and that was the problem. Bought new tires, road force balanced, all good. Vibration continues.

Mechanic wants me to bring it back in to keep trying to diagnose it. I'm done with this shop now though. I am now $8000 into fixing this and I really can't afford to keep throwing parts at it. Yes, all the parts they replaced were worn and had issues, but I really need to fix this specific issue.
I am thinking-
-Unbalanced drive shaft as it was used so no idea if it is balanced.
-U-joints weren't replaced, so maybe that?
-Rear wheel hubs
-rear axle maybe bent axle shaft or flange
-rear brake drum imbalance or bent

Any other ideas?


For those of you that want the whole story or maybe just so I can vent.....

Purchased the van from a small dealer in utah, basically sight unseen. I know dumb but it's hard to find the AWD vans. It looked clean and had a good carfax. Spent most of it's life in Arizona and Utah. Flew in and looked at the van and it looked good. In really good shape, no rust, interior is excellent. No obvious reason to not purchase it. Obviously I will now have a mechanic to a presale inspection but I didn't. So I start driving home. 500 miles into the trip, out of no where a bad vibration started. I thought it was just road noise from a bad road. I was starting up a mountain pass and the roads aren't great. After a few minutes, I can't stand it anymore and am thinking there is no way this is road noise. So I pull off and look around. Nothing obvious other than it looked like there was a spot on the wheels where some weights were at one point and were no longer there. So I'm thinking great, some weights flew off and it's just out of balance. Meanwhile weather gets worse and highway closes. So I find a hotel. Next morning is Sunday, nothing open, so I find some wheel weights at an auto parts store and try slapping some onto the same spot where it looked like they fell off. I start down the road again, no change, still bad. I need to get home so no other option but to get off the highway, take side roads and keep my speed below 50. So the 3 hours that were left on my trip turned into 8 more hours.
I get home and I'm still thinking and hoping that tire is out of balance. So I take it to a tire shop. They call me back and say they can't work on it as the wheels have spacers on it and they won't even touch it due to liability reasons. So I get an uber back, pick it up and take it home. I'm thinking I'll just remove the spacers and bring it back. I take off the tires and realize they are not spacers, they are adapters. So I go on FB marketplace to find some proper wheels. I find some for $700 dollars that came with studded winter tires. Great. Put those on, no difference.
So now I don't know what the issue is. I bring it into a top rated mechanic. My old work had a lot of these express AWD vans for their fleet and the owner would only bring our vans to this mechanic, he also used to work there. So I'm thinking this is a good place to go. They diagnose the front hubs, control arms, tie rods, stabilizer bar links, upper and lower ball joints, and had it aligned. They stated some of these were very very bad. All of that cost $4300. Now I work on my own vehicles often. But I went to this mechanic because I figured they would diagnose it correctly, fix it, and I'd be done as I had no idea what was wrong. I would have replaced all of these things over time myself to save some money normally.
So they say it is fixed. I pick it up and get on the highway to make sure. It is better, but now once I get above 70 it returns, although not as bad. So I immediately bring it back and say it's not fixed. After awhile they believe that the rear yoke that connects to the rear driveshaft is seized in the transfer case and that is causing the problem. They cannot remove it. It is supposed to slip out and then down to remove it. It won't. So they say that I need a new transfer case and rear drive shaft. Okay... Another $3400 and they cut out the drive shaft and replace it and the transfer case. Great.
I pick it up, test drive it, I think it's good. Now I don't drive it much as it was going to be our road trip van. I took it up to the mountains to ski, I didn't notice anything. Come spring, I put the original tires back on, non winter ones. We take it on a 3 hour trip and the vibration is back, or never went away and I just didn't notice it with the short trips and the studded winter tires and the added road noise. I don't know...
I bring it back in because the transfer case has a warranty and they wanted to check everything over after 500 miles. I tell them the vibration is back, or never left. They agree it has a vibration but they they it's different this time. They think it is the tires. They are no name cheap tires that came on the van with the purchase and they think there is "feathering" on the tires and that could cause the vibration. So I get new tires for $1100. No change. They also roadforced balanced them to ensure the wheel was okay. Meanwhile it's getting warm and I figure out the AC doesn't work. So capped off the rear as that is a likely problem with leaking, replaced every component upfront and got it working. Also in the middle of all this, I go to title and register it and am told the title is not the most current one. After many calls back and forth I get the correct title from the dealer. The dealer also refunded my $1500 for the massive repairs I had to do which I know is very uncommon and helped a bit, but not really.
So I get the new tires, get on the highway, no change. Vibration is still there. Then out of no where it sounds like I have no muffler! I get home and see that the pipe going into the muffler is disconnected, like the weld just gave up. No rust... I don't get it. So I have ordered a replacement to put on. I also had a squeaking belt so I replaced both belts and idler pulley.
So yeah... thats the story. I'm sure I'm forgetting some other things.
Would greatly appreciate any help or ideas.

 

Just wanted to update with a bit more information. The vibration is there whether accelerating or not. It almost seems as though the vibration continues to a lower speed, maybe 60-65 once I have it up above 70 and then coast. Like it doesn't start until 70 but once I reach that speed and then coast it continues down at a lower speed for a bit until I return back up to 70. I have not tried dropping it into neutral at that speed to see if that changes anything. I will once I replace the muffler. I also read that I should give it a slight sway left and right at speed to see if it is the wheel hubs. I also read that i could jack it up and spin the rear wheels to see if they wobble the slightest bit and that could be bent axle shaft or flange.