With the 'Super Select' system found in the Gen2's you can engage the factory rear locker while in 4 High, locked center diff or 4 Low, locked center diff. You would have to fiddle with the wiring to have the rear locker engaged in 2 High. Normally I run in 2wd, but if the conditions are lousy I run in 4 High (essentially AWD with the center diff in viscous mode). In short, the Gen2 offers:

2wd High
4wd High (AWD)
4wd High Lock
4wd Low Lock
4wd Low Lock and rear locked (if you have the factory locker)

If you have an ARB I suppose you could lock it whenever you felt the need.