Just another thought on this vibration problem?
Other than the Range Rover 90 Classic and the 93 model Discovery, I can remember having a 1975 109 LWB many years ago and it develloped the same type of vibration but I never found the reason for that either.

There has to be others who experience this vibration that feels like a worn out universal joint in the front prop shaft.

If you ever found what caused this problem and you managed to get it repaired, please let me know what the cause was. It is driving me crazy.
Both our vehicles have the same problem and I have just purchased two new prop shafts to see if that would remedy the problem, and it did NOT.

Best wishes,
Shirley...