A full-time 4WD vehicle also has a center-diff in the transfer case, because the front wheels rotate more quickly than the rear ones when turning a corner and traveling forward.
A differential always:
- distributes equal amounts of torque to both wheels
- reacts to resistance/traction to allow a wheel with more resistance to rotate less and a wheel with less resistance to rotate more (The rpm difference created by the differential is always proportional. If the inside tire rotates 15 rpm less in a turn than going straight - then the outside tire will rotate 15 rpm more than going straight.)
Locking the differential is accomplished by doing one of three things:
- Installing an open differential
- Installing a locking center section which ALWAYS locks the differential
- Installing a "lock-on-demand" unit like an ARB air locker