Dan:

Usually the rear one lasts a lot longer than the front one (maybe due to higher temp in the front?). However, the vibration you mentioned could be from other sources as well (center bearing, differential, differential mounts, etc).

My mechanic mis-diagnose a slight clunk I had in my 1991 300E. After replacing the front flex disk, the clunk was still there. Finally he narrowed it down to the differential. Now I am using a heavier oil (Redline synthetic 80W140) and the clunk (along with a whine) is not noticeable.