Okay I have another theory. How is driveshaft length? It wasn't a tight fit was it? Reason I ask is off the driveshaft needed to be compressed a little bit to install, you wouldn't notice it at certain speeds/rpm's, but at others torque could cause the driveshaft to become slightly twisted and take a spiral shape. Much like when the rubber isolator goes bad on the driveshaft center bearing, it isn't noticeable at certain sites, but others it bangs around in the housing and causes vibration.

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1982 300D Turbodiesel, daily driver. Mods so far: Fram 8038 paper filter, 4 brake light mod, Gen II w126 (front) rotors/calipers, boost turned up to 12lbs, non-egr manifolds, water/methanol injection, 4-speed manual
1980 300SD Turbodiesel, project car, nearly ready to hit the street

1974 240D, New paint