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It seems rather obvious to me that the solenoid operated clutch that controls the power delivered to the rear differential should have been designed to transmit less power than is required to break the differential casing. It would be much more sensible to allow the clutch to slip if excessive torque is applied rather than cause a completely disabling breakage.
Update from Jul 6, 2017: I have since taken the solenoid operated clutch off and it was apparent that the pinion was very loose in its bearings. The failure was due to the retaining nut coming loose and allowing the pinion to be drawn into the rotating differential pinion cage on the overrun.
There is no mechanical locking on the pinion shaft retaining nut. It has a left hand thread and can only be checked for tightness by removing the solenoid operated clutch. If it comes loose a juddering on deceleration could well be apparent before the differential casing breaks up very soon after. If I had stopped driving the car as soon as I perceived a problem it may have been repairable but I was towing a trailer and thought at first that the juddering was probably due to the trailer. Also, there are often ridges in the road surface approaching roundabouts here, which was not helpful in this case.
Without doubt, the pinion nut should not have come undone but I hope my experience will be helpful to Nissan X-trail owners; forewarned is forearmed, as they say.