Hey. Im having issues with my diffs. When I put my truck in 4x4 it works fine at low speeds in forward and reverse. Between 15 and 20 mph the right rear tire starts to drag and the truck feels like its ganna rattle apart. I ran it on the rack in 4x4 and it worked fine.

 

Sounds like something is binding in the transfer case. If could just be failing but if the truck is new to you then check that the gears in the rear and front diffs match. Running larger tires on the rear will also cause problems.

 

Sounds like @burnhedge has something to consider, he mentioned that your gear ratios may be different front to back, like someone rebuilt the differential at some point in time? If they did, I would assume it was the rear diff, but we all know what assumptions make you and me... So, what I might suggest, especially since you have already had it on a rack, is get the truck back on their, and find a way to measure the speed of the front axle, and the rear axle, this may or may not be difficult, because with a differential I don't know, because I have not done what I am suggesting, but on each axle their is a chance that each wheel could be spinning at a different speed. What I am thinking is you get the truck in the air, in both 2 wd and 4 wd, to see if you can see, hear, smell any changes between the two, but to me most importantly, I would consider wheel speed, to see if they are different. Again, I have never done this, but I personally would start by comparing the axles to themselves, left to right, so you know if in fact you do have one side on an axle go substantially different speed than the other, which I don't suspect to be the case, unless something is internaly wrong with the differential, but I can assume you would know by now if you had a bad diff. Now on measuring wheel speed, you may be able to see this with your eyes, but I would not count on that. What I am thinking is one of those lazer operated tachometers, that you point at stuff and it tells you how fast it is spinning. If memory suits me though, they usualy like a definitive thing to count off of, so you may have to get some chalk or maybe even paint and put a good healthy stripe on the bottom of each tire, I would suggest keeping them the same sises on each tire here, because you don't want to build problems right into your own test. And thinking about it, I am assuming here that their is no speed diferental in the transfer case since, unless you are special, is a part time system.

I hope this made sense, and maybe if we are lucky, some other people will come onto this here thread and pick apart my suggestion, because I am only old enough to still be in high school so, yea... Good luck!

 

What I’m suggesting will happen when someone has their rear diff fail. They run to the salvage yard or Craigslist and get the first complete rear end they can find so they can get the truck back on the road. If the gears in the new rear don’t match the gears in the front it will bind up the transfer case.

You can get a good idea if your gears match by spinning the tire a full rotation and checking the number of driveshaft rotations. If your glove box codes are still there you can pull the rear cover and check the stamp on the ring gear and see if it matches the glove box code , it’s unlikely that the front diff has been replaced.

Of course if the OP has had this truck awhile and never had issues in 4x4 before this is a mute point.

 

@burnhedge all good points

 

I get what you’re saying but it’s just not as simple as getting the wheels off the ground and measuring wheel speed. The reason a differential is needed is to allow wheels on the same axle to spin at different speeds and that only happens when there’s more drag on one wheel than the other.

The overall condition of the axle assembly (meaning diff, axle shafts, bearings, seals, and fluid level), brakes, and wheels will all affect how much drag is on each side, and even small variations will allow one wheel to rotate faster than the other. To accurately check wheel speeds this way, you would first have to measure the rotational torque of each wheel and compare it to the opposite side to see if they’re the same.

If you’ve ever been stuck in mud, ice or snow and had one wheel spin while the other wheel doesn’t, that’s the differential allowing it to happen. Most people don’t know this, but if you partially apply the parking brake, it equalizes the drag on each wheel and keeps them spinning at the same speed, then you can drive out of whatever you’re stuck in. Limited slip and locking differentials work on the same principle by preventing the axles from spinning at different speeds.

IMO if you’re unsure about the diffs being the same, the easiest way to check is to remove the cover and read the stamps on the gear.

 

Defiantly a sure fire way to find the ratios, I would think to it would be the rear diff that may have ever been rebuilt since I don't imagine people need to change out front diffs often.