Well, not much to say that hasn't been said. The lockers are called Vari-loc by Jeep, Hydra-loc by the units original manufacturer. There is a clutch, piston and oil pump inside the case. When one axle spins faster than the other, the pump produces pressure that applies pump pressure to the piston and then apllies the clutch and locks the diff. Very similar to a trac-loc with the clutches, but will release and not cause steering drag. This unit will not totally lock up, but is better than an open differential. The D30 in the front of the WJ's is slightly different than the average XJ D30, it is machined to accept the oil pick-up plenum that is attached to the ring gear side of the carrier. The axle size is larger, not sure about the spline count though. Custom axles would be needed to run this unit.
The Jeep website has some basic info on the Quadra-drive system.
http://www.jeep.com/grand_cherokee/4_wheel_drive.html?context=grand_cherokee-index&type=left&mtNav=0
You would be much better getting a drop in locker or a full replacement locking carrier made for the XJ D30.
Bryan