When reinstalling the driveshaft, it is important how each bolt, nut and washer is oriented. Also, does your driveshaft have the vibration dampner? Is your flex disc mounted correctly? If the mounting details aren't correct, a new driveshaft isn't going to solve the problem.
Here are a couple of links to help:
Mercedes-Benz Model 123 - Disc 2
http://www.peachparts.com/shopforum/tech-help/235575-flex-disc-orientation-safety-durability-issue.html