Well it definitely sounds like something is loose or worn out. Does the vibration follow vehicle speed or engine speed? If it follows vehicle speed I would suspect something in the suspension or steering. If it follows the engine speed I would suspect something in the drive train.

I would crawl under it while having a buddy turn the wheel back and forth, and move all the suspension and look for any play in anything, (bushings, tie rod ends, loose bolts, a broken spring, etc.) if I suspected something in the suspension.

I would check each u-joint, and make sure all the lug nuts are tight, and if they were all good I would put the rig up on 4 jack stands and put it in gear, let it idle and see if anything was moving or vibrating that shouldnÆt.

Sean