I would be looking to the transmission more than the transfer case or output shaft. There is more torque when starting off than when cruising in fifth, so transfer case or output shaft problems show up more so then. Also, you say that the vibration stops when you accelerate, and I would expect it to be worse then.

Also check wheel bearings. It appears to only happen at one road speed from your description.

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2007 X3 3.0si, 6 MT, Premium, White

Retired:
2008 535i, 6 MT, M Sport, Premium, Space Grey
2003 X5 3.0 Steptronic, Premium, Titanium Silver
2002 325xi 5 MT, Steel Grey
2004 Z4 3.0 Premium, Sport, SMG, Maldives Blue