If you feel it in the tranny and only at highway speed it could also be your rear going south. That's exactly how my '84 acted when the rear went. However, that was a 7.5" rear, which was notorious for it. Haven't heard the about the 8.8 doing it as much. It's pretty easy to bend a wheel though, and a back wheel out of round would cause a drivetrain vibration you would feel in the shifter, especially if it isn't the stock shifter with the vibration isolator bushing.