Well yikes, I've had this truck 2.5 years, for 30k miles, now at 191k. Over that time on some stretches of concrete highway, the back end has developed an extreme vibration at normal freeway speeds - enough to shake off the center hub caps one day when I "pushed it" instead of slowing down, to see what would happen...

It only happens on certain stretches of concrete highway, the cracks between the concrete sections seems to be the main changing factor. Some stretches of highway are worse than others, different distance between the cracks & probably different wear on the road are probably the changing factor. Black top, even blacktop over concrete, no problem.

Usually starts in around 60 mph but I think that's slowly dropping. I slow down to 60-65 to a bearable level of shaking when it happens.

Adding load to the truck makes it worse; sandbags in winter brings it out some, I added a topper 6 months back & it got quite a bit worse, esp when the sandbags are also in the bed. ...I suspect without the topper & sandbags, it probably had a smaller tendency to bounce as it got worse. It had no bounce when I first got it.

After it developed, I balanced & rotated tires - very very slight difference, maybe 6 months back. Underinflated tires make it worse.

I also changed out the shocks; over the 2.5 years I owned it, the shocks seemed to go from fine to slightly worn out with a bit of slow bouncing after bumps at freeway speeds; I put on good Monroe Spectrum shocks. That fixed the normal worn-shock slow bounce, but very little difference to the wheel & axle hop.

Any thoughts on this? I thought I'd take it in for a 4-wheel alignment & ask the shop to check carefully... I see other posts talk about the driveshaft center bearing, u-joints, & driveshaft balancing. I like doing my own work but getting old enough that's not as feasible anymore. I could certainly change u-joints myself & maybe that center bearing, but it sounds like even if I'm careful about putting the driveshaft back up without rotating it, and not spinning the rear end, the driveshaft balance would still have to be done?

Any ideas appreciated...