Front axle shaft U-joints will vibrate bad if they are worn enough. Did you check the top ball type joint on the trac bar? Have somebody turn the steering wheel a couple of inches left and right, slowly and look close at the top trac bar, ball type joint (while your there, look closely at the mount to the frame), if it´s worn you can see some movement up and down at the joint and if it´s really worn left and right play. If you take off a tire and lift the knuckle (front brake dust shield) at the bottom, you can see play in the top ball joint pretty easy, after wipping the grease off.
A quick alignment check with a tape measure, is pretty simple, "GoJeep" had a good write up on it.
Have someone turn the wheel a couple of inches left and right and look closely at the sector shaft, coming out of the bottom of the steering box. Have them progressivly turn the wheel in smaller incriments and then slightly larger increments and see how much the wheel has to be turned, before the sector shaft actually starts to turn. Could be a bit of play in the steering box, along with some other front end problems, that are adding up and/or interacting.
