If there is damage to the centre-plate or release bearing, having the clutch depressed could be stopping the play in either the release bearing or centre plate from vibrating. Crankshaft end-play has been mentioned as a possible cause---I think that this is unlikely. It is very difficult to diagnose with any certaincy from a distance, but I really do supect the clutch as the main culprit. There is a small chamce that the flywheel has become loose---again, engaging the clutch would put pressurs on the flywheel that would eliminate the vibration. I thank that this is going to call for an"engine-out" job, not a difficult job.What's your thinking here? I've never worked on a transmission before. If the car is stationary and in neutral, rev the engine to 3k, the car shakes. Drive the car in 1st at 11 MPH, or 2nd at 18 MPH or 3rd at 30 MPH or 4th at 43 MPH, the engine is doing 3k, the car shakes. Do the same road speed with your foot on the clutch, the engine is not doing 3k, the car doesn't shake. What is happening with the release bearing or pressure plate that is causing the vibration?