I have a 97 4x4 Reg cab that is completely stock except for 33x12.50x17 tires. Between 40 and 50 mph I get a bad vibration coming from the rear, I have swapped tires and wheels around and no change, changed the rear u-joint and no change, have tried change tranny rpm via shifting in and out of OD could not tell a difference either way. Any ideas about this problem would be GREATLY appreciated. THANKS

 

Dont rule out your front U-joint. To tell if you have driveline slop, block your tires and put your truck in nuetral, then turn the driveshaft back and forth by hand. There will be some play in the rear end gears but you should be able to tell if there is play elsewhere. You can also remove your driveline and have it balanced. Here in Kansas City there is a place that does it for about $40-50 with new joints. It could also be the yokes if they are worn allowing slop in the joint or not centering side to side when you put the u-joint in. If your wheels arent stock wheels, they may not allow balancing weights to be used on the outside face, so they may not be perfectly in balance. For that matter, all of your wheels could be out of balance even if weights are used on the outside face.

It could be something in your rear differential, but I wouldnt start there. Its less likely and more expensive.