X drive doesn't have a full lock, that isn't part of the design. What the clutches do is vary torque balance from front to rear, they don't lock together the front and rear output shafts, those remain open.

HDC doesn't do anything with the transfer case. It is a brake control, applied individually to each wheel based on feedback from the ABS sensors. It is just a different routine in the ABS system, with added throttle control.

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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