Car: 87 Mustang GT (sorry, not a truck, but principles are still the same). 8.8" rear end with Traction-Lok, 28 spline axles from 1996 Cobra (5 lugs) installed about 20 years ago.
Just noticed a periodic vibration about once every couple seconds while driving under load. Then coasting in neutral, a very steady vibration, strong enough to make the rear view mirror unusable. A search of the net brought up things like driveshaft out of balance, loose bolts on the driveshaft, worn U-joints, or worn wheel bearings. Being a 31 year old car with 170K miles, any of those is a possibility.

Checked the U-joints, and could not detect looseness. Can't check driveshaft balance, but no play at the transmission, and all bolts at the diff flange were tight.

Then checking wheel bearings, I grabbed the wheel on each side and tried to push them up and down, and find ever so slight motion. So the question is, is there any motion allowed?

I pulled the axles for a closer look at the bearing surfaces. Any opinions on how they look? Are they still serviceable?



left and right axles

 

Do the bearings press on to the axles? The bearing isn't riding directly on the axle, right? Unless that's the bearing surface, the look ok to me.

Take your u-joints apart. I think you will find the issue. (based on how my '65 was with worn u-joints)

 

Yes, in the 8.8" rear ends, that smooth part of the axle shaft is hardened and ride directly on the roller bearings, which are pressed into the ends of the axle housing. For cases where that area on the shaft, which appears to be only about 3/8" wide, gets too worn, there are offset bearings available so a different part of the shaft can ride on it. I need to decide which type to get.

I found no detectable movements in the U joints, but I'll check them out anyway. thanks.

 

Since the rollers ride on those, I don't like the right one in the picture.

Having said that, the vibration you described is, imo (FWIW LOL) too severe to be wheel bearings. Having driven a Ford wagon to the point of having the bearing fall out in pieces when I took the retainer off, I can tell you it was very loud, but not much vibration. The failing u-joints made vibration. They will seem tight when things are all bolted up, but in my case the crosses were terrible, and most of the rollers were mia.

 

You sir, were correct! It became obvious once I removed the driveshaft flange from the diff flange; the yoke would not move in one axis. Taking the U joint apart revealed that one of the bearings had turned to powder. Time to order new replacements.

If it wasn't for the leaky diff cover, there was no need for me to take my rear end apart. The axle end seals seemed kind of stiff, and seeping a bit more than I'd like, so they're being replaced as well. One good experience

I'm thinking that I should replace the bearings anyway because of their age and mileage, and I have everything apart. But I broke a cheap puller trying to pull the first one, so I'll have to find something better.

One good experience from this is that when I serviced the rear end some 20 years ago, I replaced the factory lube, which had turned into a black goo, with Mobil 1 gear lube. Draining it now I see it's still a nice clear amber color. I'm sold on Mpbil 1.