Somewhere there is a torque spec for checking clutch type limited slips. It was easy on the old AMC cars because the two-piece rear axles had a nut on the outer end of the axle and you could put a torque wrench on that.
With an XJ, you would have to make up an adapter to bolt onto the hub, with a 3/4" nut welded in the center, on axis with the axle. Then use a torque wrench to try to spin the axle with the opposite wheel on the ground. The accaptable number was surprisingly low, IIRC, something like 25 lb-ft. If it broke free at less than that it was deemed to be worn out.
Those numbers were posted some time ago -- don't know if a search would turn up the thread.