All you with this problem, it more than likely is unbalanced tires, often but not always occuring around 60-70 mph. If you can feel it in the seat, it's in the rear, steering wheel= fronts. Try swapping tires fron to back to see if it changes. Otherwise I'd say ball joint(s) or tie rods.

 

This is what i was told by discount tire they said it was common to have to balance larger tires several times in the first 5000 miles ive had mine done twice in the first 1500 miles it took care of the problem both times

 

Think I figured mine out, it was loose TPMS sensors installed incorrectly on aftermarket wheels.

 

I ordered a FabTech stabilizer kit for my truck, it would not fit. Not sure if it was because of the Superlift kit or not. I have had this happen also. My Ebrake cable was froze and slightly on, I do not use the Ebrake but have bumped it climbing in my truck. The whole truck would shake terrible. I will be putting in a 9.75 rear end with rear disc brakes and checking every suspension part this spring. Will see what happens.

 

I have the same thing on my 99. im thinking about a single steering stablizer