All the differentials are open. If you spin one rear wheel, the other can just spin the opposite direction, if the drive shaft is fixed. If you spin both rear wheels the diff in the transfer case is open anyway.

It isn't commonly referred to as 4wd, but rather as awd, meaning the front and rear are not locked together.

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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