It's the automatic locking rear differential option. An Eaton unit you should be able to find a lot of information by Googling for "Eaton G80".
Basically it's better than a limited slip differential, but it can be fragile if abused with full-throttle engagements. When it detects one rear tire slipping, as soon as the difference in RPM between the two wheels reaches 100 RPM, it will mechanically lock up the rear wheels so the torque will go to the one that still has traction and not to the one that's uselessly spinning on ice or snow or in the air.
It's the device we have that makes lumpy hill climbs like this possible:
[video=youtube;24SwmSN0-bI]http://www.youtube.com/watch?v=24SwmSN0-bI[/video]