Likewise, a 4WD vehicle is only two-wheel-drive, one front and one rear. Many trucks up here have limited slip differentials in the rear so they essentially have three-wheel-drive.
Many 4X4 competition vehicles have "lockers" or "spools" which defeat or outright eliminate the differential, since they are trying to get maximum power to the ground, and are spinning much of the time anyway.
(See: "CORE" and "Forumla 4X4 Hillclimb" on youtube)
Helpful Hint:
When stuck in the mud/snow, etc., you can "fake" posi-traction by partially applying the parking brake.