I had the same problem. It would start to shake violently when I was accelerating.

Ended up being the U-bolts that were holding the lift blocks in place on the rear axle. THey were loose and my rearend was wobbling!!!! Needed to take it to a shop and have them heat the bolts and impact them tight. Luckily the rearend was not harmed.

If you have a lift, especially if you have blocks stacked on top of eachother (stock lift and another lift block). THey will shift and make the U bolt loose. Had to get them retightened twice.