Expedition I 1997-2002, II 2003-2006, III 2007-... -(Edit)
Two wheel drive standard, optional Ford's Control Trac selectable all wheel drive (2H - Auto4wd - 4H - 4L). In Auto4wd mode the behaviour is similar to the automatic all wheel drive - the vehicle is normally rear-wheel drive. Torque is transferred to the front axle via an electro-magnetic clutch when slipping occurs. In 4H mode the clutch is fully locked and the front and rear driveshafts turn at the same speed. Optional locking rear differential.
Figure: Ford Expedition (2003) vacuum-disconnected front hubs for 2wd mode

See http://en.wikipedia.org/wiki/Control_Trac
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