My 99 scab does the same thing. Hate to tell ya this but its the tranny. I first thought it was u joints too but I put all new in and it still does it. But hopefully for you its not the same problem. But on a side note cause I don't have the money to get mine fixed, do to all the other mods I have done its still running strong with over 300k miles and it started around 225k.
i was gonna say i know ujoint problems and its at certain speeds not just reverse the angle of my axe in different "positions" has lots to do with that