I would say that the fact that there was a "real" change with the new shaft is a good indication that the problem is there.
MB has always placed great emphasis on not changing the relationship of the front to rear sections of the shaft. I have often wondered why they sell just the front and back halves. Buying a complete shaft assembled and balanced by MB should solve the problem.
Unfortunately, the other possibility is that you had two vibrations and only fixed one with the driveshaft. Your car doesn't have the miles to have problems with any of the joints, but I see as many half shaft vibrations as I do driveshaft vibrations.
Cars like yours that have sat for most of their lives can get some real weird problems.
Bosch Master, ASE Master, L1
33 years MB technician