Rear wheel bearings can go bad. Not as common as front wheel bearings, but it can happen. They are pretty easy to check.

 

I had to replace both rear bearings and axles at 200,000 miles. There was a notch in the surface hardening of both axles at the bearing contact point.

George

 

Good thing to bring up, if you're digging into the rear, best to replace both rear bearings while you're in there. The work required to do both is only marginally more than it is to do one and the parts aren't really all that expensive. One thing you will want to determine before you get too far into it is whether your axle shafts have been scored by the seals. If they have, there are alternative bearings/seals that move the location that the seal rides on to a different spot.

 

i considered those repair bearings for my situation but I read instances of failures of the extended bearing assemblies so I bit the bullet and went with a “proper” repair. What is your experience with these bearings, are they any good?

George

 

Depends on what you're getting into. If the axle is worn enough that you need to move where the bearing rides as well, then you really should be looking replace the axle shafts. The ones I have seen in the past just move the seal location. Admittedly, it has been probably 10+ years since I have replaced bearings on the style rear axle that the s-series trucks use. All of my work trucks have full float axles now so they're completely different setups.

 

I had done rear wheel bearings on a '00 Silverado before is it the same as that? Would old/watery rear end lube cause a noise too or would that just eventually cause catastrophic failure? The biggest thing that still drives me away from thinking bearing is that it ONLY makes the noise when the drivetrain is under load. If it was a wheel bearing it should make the howling regardless of if I'm accelerating or not right? Could it possibly be the pinion bearing in the rear end?

 

Pretty much the same process as the full size trucks. I doubt that fluid condition alone would cause a noise.

Another thing to consider is a failing universal joint. They can make noise under load or sometimes when coasting depending on what's going on with them.

 

Am I correct in thinking that the front driveshaft should not be turning at all when in 2x4? The sound really sounds like its coming from the front end. If it was a failing U joint I would think the noise would be coming from the rear when in 2x4 since only the rear driveshaft is spinning then. Unless I suppose the front U joint is probably far enough forward it could sound like its coming from the front end. Do these trucks have a shorter tail 4L60 than silverados? On my silverado the front U joint was almost at the back of the cab and wouldn't have sounded like it was in the front if it failed.

 

On a 96, the front driveshaft should spin freely when 2HI is selected. It could still spin when driving due to parasitic drag in the various drivetrain components, but it should not be powered.

I mentioned the u-joints simply because of your wife's observations may have shifted you to the rear of the vehicle and only based off of that statement. At that point I was just throwing out ideas for you to look at and dismiss as possibilities through actual hands-on diagnosis.

 

It doesn't help that I'm usually hyper sensitive with these type of things on my vehicles. I hate when things make noise when they shouldn't (or don't make noise when they should). I was a USAF Aircraft Mechanic so to me any noise is a bad noise because on an aircraft even a small noise can kill you real good. That coupled with my ME degree knowledge of vibration fatigue in machine design is a death sentence for playing out cataclysmic failure scenarios in my head when I hear a noise... Chances are this vehicle just makes noise and its fine but by God I cant stop looking for what it is.