You could probably track down the manufacturer of the stepper motor, and see if they are available direct.
You may also be able to repair it, if t is gummed up or rusted. 4-bolts and electrical connector to remove...
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I have had the rear locker jam up on mine. noticible when driving around town with a locked diff. I changed the oil which was black and dirty, reversed the car and it gave one final bang as the diff released and hasn't given any trouble since. I changed the oil as a precaution 12000KM later and it was clean. This happened at around the 200,000 Km mark. Not sure if the dealer had changed the oil at 130K as they are supposed to but I couldnt find any reference to them charging for it:no2:
Anyway. If the diff appears to be locking an oil change is cheaper than a new actuator.
Regards,
Tote