I have no problem with them trying a driveshaft to verify a diagnosis. My problem is having you pay for anything more than the R&R time if it doesn't work.
BTW most of the driveline vobrations we see in 123 body diesels are in the drive axles. Recently we have found that switching the axles side to side gets rid of the vibrations. I first tried this after the Rolls Royce dealer I deal with told me to try that on a 1980 Silver Shadow. The Silver Shadow (somewhat like the MB) has changed their axle design and it would have taken a small fortune to replace the shafts (same condition with MB factory shafts - they have been redesigned). Since fixin the Rolls we have done the same trick on four different MBs (one 107 chassis and three 123 chassis). We do not do this with axles that are obviously bad.
Bosch Master, ASE Master, L1
33 years MB technician