Just went through the same issue on a 57 1/2 ton. After the wheels and tires were balanced again and the loose wheel bearings in the front were replaced, a little but not significant improvement, we were stuck with the rear end. So per my usual start with cheap and easy...yeah that was working we pulled and replaced the u-joints and..still there. Last resort? Took the drive shaft to a good shop that only does this kind of stuff. The result? The spline joint was worn badly. They took it all apart. Welded in a new one. Straightened, balanced and it is now smooth at all speeds. I did put new seals and bearings at the rear axles just so I would not have to get under this truck again for a while. The owner is happy but I wish that I had found it sooner.
