One thing to check when re-newing the rear flex disc is the centering pin a.k.a. the Driveshaft Support. It centers the rear driveshaft onto the differential, contains a rubber ring that holds grease inside. Likely the rubber ring has worn out as it has on mine.
Replacement requires removal of the driveshaft, a good time to do your center support as I should do mine. (vibration at 35-40mph, no load, from center of car) on mine.

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63 220S W111
76 300D W115
2013 VW JSW TDI M6

previously-
73 280 SEL 4.5
86 300E 5 speed
2010 VW Jetta TDI M6