[The Fixer]

Sounds like Death Wobble - could be the hub bearings like Justin mentioned, or also bad bushings in the control arms, or tie rods, or trackbar, or....you get the idea. A good way to find where the loose parts are is to have and someone else turn the steering wheel back and forth while the truck is on the ground and you look for any play in the steering setup. I did that with my brother-in-law on his '04 Grand Cherokee (very similar steering setup to your TJ) and we found his trackbar was shot. He could turn the steering wheel almost 1/4 turn before the trackbar would move, and that was causing his Death Wobble.