…there is a difference between mechanical locking fidderential and viscous differential. Most passenger vehicles, SUV’s and light trucks are viscous clutch plate type diff’s. Some Audi’s and Subaru’s have mechanical locking differentials. Subaru for instance, depending on model(STi, for example) has a electronic adjustable center diff and 1 each front and aft(3 total). Audi Quattro’s uses Torsen center differentials and some of the transverse mounted engine cars uses Haldex electronic locking diff’s(also found on some VW’s). When locked, it is a true 4WD.

Also maintenance of the diff’s makes a difference as the clutch plates do wear out. Since it is rather expensive to service the diff’s, most owner hardly ever have them service during the life span of ownership.