[olddog]

It is seeming to me that it is a shaft that is not necessarily out of balance, rather not being held centered and allowing it to wobble off center of its axis of rotation. Like a bad bearing, worn shaft. Even a pilot bearing that the input shaft is loose or too short to reach it.

I had an industrial blower with a loose bearing on the shaft that the vibration was worse when slowing down than it was when speeding up.

Any way if the car is in gear and stopped with the clutch in, the input shaft and clutch will not rotate. However in nutral clutch out the shaft, and clutch will rotate. Try to simulate the rpm coming down as if driving.

On the pilot bearing, if it were loose, every time you push the clutch in and let it back out, the alignment could change. Does the vibration seem to randomly get better and worse as you shift?