I know on some of the older Mercedes the crank and the flywheel were balanced as a unit. I have balanced flywheels before. You need a lot of special equipment, so it would need to be done at a shop that works on driveshafts and such.

What I am saying here is that perhaps the problem with the vibration is coming from the change over to the 240D manual shift flywheel from the 300D auto shift flywheel.

I know what type of job it would be to tear down the car to do this, but you might consider trying it out on a future conversion, or if anyone else here does one they could try it out and report their findings.

If you have the equipment it takes about 10 minutes to balance a flywheel/ring gear combo so it should not cost that much if the flywheel is out of the engine.

Pooka