Then you have the x drive version (2004 onwards) and it has a variable torque split in the transfer case. There is an electric actuator on the transfer case to control the torque split. If you unplug it at the case, you will have disabled AWD, and have RWD only.

Out of curiousity, why do you want to?

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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