And just as a side... you're not at fault for saying they "lock". When functioning properly these diffs do provide full axle "lock" (ie not allowing differential wheel speeds).

From: http://www.eaton.com/EatonCom/Produc...disc/index.htm

EGerodisc™ is a hydraulically-operated electronically controlled limited slip differential that provides variable torque up to full axle lock. It automatically identifies the optimal traction solution at any speed. The EGerodisc™ is a commercially proven technology in production on the Jeep Grand Cherokee. It provides superior vehicle cross-country mobility and improved control for safety and roll-over protection. The electronically controlled actuation provides rapid response for stability and traction control. It eliminates driver interface; no driver training is needed. The EGerodisc uses standard gear oil and is virtually maintenance free.
Good video showing how it works: