There have been a few postings about clutch grabbing/chattering and shaking.
I had a vanagon that did this too, but would go away and be good for months
at a time.

What is happening: A clutch can be considered to be a system like a wheel
suspension system. Where a wheel has a spring, damper (shock) and mass
(either the car or the earth) and has to have the right amount of damping
and spring rate to make the mass act nicely.

 A clutch has the same things: the springs are the springs in the clutch
disk and maybe windup of the drive train. The mass is again the car or
earth. The damping is much harder, newer cars (golf II/III) have hydraulic
dampers built into the motor mounts, older vanagons have no designed
damping.

Porsche, BMW, and Audi have dual mass flywheels that have dampers and
springs in them

Anyway, When the clutch is engaged there will always be some amount of
torque variation that CAN make the car shudder and shake. If there is the
right ratio of damping and springs (called transmissibility) this vibration
will not be transmitted thru the drive train to your butt.

If the transmissibility is wrong and the amount and freqency of vibration is
right then an incredible amount of shaking can be generated. But just at a
certain frequency!

What can be done:

1. Reduce the clutch torque variation, Get good clutch parts and make sure
that they are true and flat.

2. Make sure that the parts are and stay clean. Oil can do real bad things
to the frictional variation of a clutch disk.

3. Make sure that the motor mounts are in good condition. This may mean
replacing mounts that visually are OK. Rubber parts can change their
properties over the life of a car.

4. Make sure that the loosness of the drive train is OK. Worn CV joints or
drivshafts can make the effective spring rates of the drive train change.

5. Try changing the speed that the clutch pedal is let up. Not too quick
though.