Hi! I just bought a 1990 GMC Jimmy. It has 54,000 miles original, but had some of that lovely Ohio rust. Anyhow, just recently, the transmission has developed a shudder when in a forward gear. It does not seem to do it at all in reverse, but when I shift into Drive and hold the brake, it has a shudder. It also does it when I lift the accelerator pedal on the highway at high speeds. I did a tune up and the engine when I got it, and the motor runs really well, so I can't figure this out. Anyone have any clue?