I had a bothersome vibration on my '85 300CD (142K miles) that about drove me nuts. Tried everything to correct it to include balancing main drive shaft, new flex discs, new engine mounts, etc. Finally replaced half-axles and that solved it. Mine weren't leaking oil but something was wrong with them.
Apparently, vibration can be caused by many things and trouble shooting is hit or miss. I took my car to any number of shops (including Mercedes dealer) without success.