[1989GT]

Do you have an adjustable cable and quadrant? If so I would play around with it to see if the clutch is adjusted right, since the vibration only occurs when the car is in motion I'm leaning more towards the driveshaft. Reving in neutral(clutch pedal out) would rule out the clutch/pressure plate/flywheel and input shaft on the tranny. I would disconnect the drive shaft and rotate it over a hole and hook it back up and see if the vibration is there/get's better or worse. Do that all the way around until it either stops or you're back at the starting point. I would say though that it could also be the clutch is not adjusted right and could just be manifesting itself while in motion at 3k RPM.