Steering Wheel Shake

I have an '04 Armada SE Off road, and although I love the vehicle, I have had problems galore. The newest problem, that they cannot seem to decipher, is an obnoxious steering wheel shake when I drive on the freeway at high speeds (55 and up). This is not the shake that I had before the brakes were replaced (2 times), rather, this is a shake that I experience when accelerating. I do have aftermarket rims, and they are trying to tell me that those are the problem, even though i had the same shake before I put them on. Oh yeah, I also reported the shake when i had the stock wheels on and they told me that it was normal with the SE-Off roads to have that kind of shake because of the big tires. Are any of you guys having this same type of issue? Also, have any of you had success with the buy back and what were the end results? I really appreciate any help that you can give me. Thanks.