I'm getting the same type of vibrations from my 98' Bravada too. I just replaced a CV joint, as well as an idler arm, both lower control arms, and a few other things. That was about 2 months ago.
Just last week it started vibrating, but it isn't related to engine speed at all. I can shift to nuetral and as long as I'm going over 35 it vibrates. I took the fuse out for the center diff and it still vibrates.