just because you push in the clutch doesn't mean the trans. stops spinning you still have the driveshaft moving,don't you?

 

If its vibrating to where you cant drive the car, its the flywheel imbalanced or MAYBE throwout bearing damaged

 

Let's assume you are not moving. Car is sitting still. Wheels, axles, driveshaft is not rotating. Engine running with the clutch pedal up.

So the crank, harmonic balancer, eng accesories, flywheel, pressure plate, clutch disc, transmission input shaft ALL spinning. If you have a vibration during this situation.
You can now press the clutch pedal. The pressure plate releases the clutch disc, it will stop spinning in a couple seconds and so will the input shaft of the transmission, so that is what I meant.

Now with that all said. I highly doubt something in the transmission can cause a vibration and still work properly. If the transmission shakes the car it's WAY OVERDUE for fixing.

 

if your flywheel, balancer, and crankshaft is at a 28 oz. balance. then it could be the ride height of your car. but if u havent lowered or lifted it then check your u joints or maybe your drive shaft ( oil or dirt build up, dented, or off balanced ). but if its not any of the above.... then sell and rebuild !!!!