On the W126's the driveshafts do go bad, I have seen a few people vibration problems. Some can be fixed with new engine and trans mounts, as well as a new center bearing and mount. This brings everything back into alighnment. Does the R107 have a center support bearing?

If the flex disc doesn't fix it, I'd do all the mounts, and if that doesn't work, have CT Driveshaft take a look at it.

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2016 Corvette Stingray 2LT
1969 280SE
2023 Ram 1500
2007 Tiara 3200