2002 Chevy Express Rear Differential Noise
 

At about 42-43 mph the differential starts to make a humming noise.

When I lift my foot off the gas pedal it immediately goes away.

Any thoughts on what it might be?

Most likely bearings.

But check for play in the driveshaft joints before you assume it's the diff.

I just checked and no play at all in the u-joints.

When you say bearings ... pinion?

center carrier bearing of the driveshaft if it's a two piece unit

Had a similar issue with a Ram I own. If it only hums when accelerating and goes away when coasting/foot off gas, it is the pinion bearings. They will probably need to be replaced. Replace the carrier bearings as well. Probably looking at $800-$1000 at a shop (maybe more). Unless you have the tools and technical know-how this is best left to a qualified trained mechanic. The real tricky part it setting the lash when reassembling.

Yes, that is exactly what happens but doesn't make the humming noise till I hit 42-43 mph. I have zero experience when it comes to this. I texted the mechanic I use and he told me $350 + parts. The problem is that I wanted to sell the van and really need to spend as little as possible at this point. My other thought was to try and find a complete rear axle cheap and just switch them out.

R&R with a complete rear axle may be cheaper, but I'm not sure. Looking back it was ~$270 in parts (Pinion bearings, carrier bearings, seals,etc) plus almost $500 in labor, but that was almost 10 years ago. By the book it is a 4-5 hour job. You'll need to investigate how much a complete axle plus the labor would be.

If it's 4-5 hours, I bet I can find someone to do it for less than $350. There's plenty of hungry mechanics around here.

I guess I need to check prices on parts and figure out which way to go.

I would do the work myself swapping the axle out. I would just need some help humping it around.

Good luck. Around here the going labor charge by most shops is about $110-$125/hour.

What model express do you have? 1500/2500/3500? 135 or 155" wheel base? How many miles on it? Do you know what model rear axle you have? If it's a 14bolt AAM/GM Corp then the pinion bearing swap is much easier as you can pull it out as an assy, that keeps the shop labor hours down. If it's a Dana 60/70 then it will be much higher. Just because you got "no play at all" on the u-joints doesn't mean anything, if your van is high mileage and has a two piece shaft I'd bet the center bearing is shot.
One last option if the pinion bearings are the source of the noise is to check the running torque of the pinion, using an inch pound torque wrench with the tires jacked up and brakes backed off completely you should have approx 12-15inchlbs of force to turn the pinion, if it's much lower/less than that you can sometimes tighten the pinion nut a bit, it can make up for slightly worn pinion bearings/crush sleeves.