Don't know if this is even an close guess but, a while back I was riding in my brother's 96 burban, and driving it it shook like mad!! There was what we started to call a "butter zone" where if you kept it within a certain speed range it didn't shake at all. The first chance we had we stopped and I got out and checked out the tires, shook the wheels and whatnot to make sure that it wasn't a wheel bearing heading south and everything looked fine. So off we went to our destination, I told him to take it in a have it aligned, so he did. Turns out we had lost a wheel weight and the thing was now so unbalanced that by the time we had driven 250 miles on the just about brand new tire, that he had to replace it! Just an idea, but I'm sure a new tire would sure be a lot cheaper than a tranny or transfer case!!