4. When I put the clutch in, the vibration stops. When I rev the engine with the clutch in, it seems smooth and no vibration.
So you're saying if going 40 mph and you disengage clutch and rev the engine, the vibration is not there? If yes, that to me that points to a mis-aligned clutch disk; or possibly a mis-aligned transmission to bellhousing and/or engine connection. As FAST55 said, could also be a out of balance clutch/flywheel; but I'd say the mis-aligned disk would be the first thing to check. If you can safely put the rear axle on jack stands; do that and slowly engage the cultch and see if the vibration returns as the clutch engages. I doubt it's driveline/rear end related; as these components are still turning when coasting with clutch in and no vibration. Is the SM420 front main shaft bearing in good condition; if sloppy that could cause a vibration under power. Worn pilot bearing a possibility as well. Hope this helps.
