I have an 83 300D and had the driveshaft replaced at 100K miles because of vibration problems. The dealer tried everything else you mentioned, but the new aftermarket driveshaft did the trick. As I recall, MB wanted about $900. for theirs but the aftermarket one cost me about $350. That was 60K miles ago and I haven't had a vibration problem since. Not sure how a driveshaft gets warped/bent/out of round.