I say take a hard look at the rear spring perches, shackles, and axle U-bolts to make sure nothing is broken or loose. Jack up the rear of the truck to make sure it all stays connected, and you could also block the rear axle off the ground to see if you can reproduce the problem.

 

ya know what i messed around with the adjustment **** on my right rear drum today and it stopped bouncing atleast for now, i dno if the brakes were too tight or if those shoes are the problem and they arent touching now, are the brakes on these trucks self adjusting?

 

no they can be adjusted