u-joint failure causing rear differential damage

What is your question?

We had an unusual noise at 50 mph so the car went in to dealer for inspection. Were told the rear u-joint at driveshaft to differential ceased and sheared two of four differential coupling bolts resulting in a $3100 repair. Service department can't seem to explain to how this could happen and claim they have never seen this before. Car was in for service at 112,692 miles and had "front and rear differential all wheel drive service required maintenance" Now at 121,+ a major failure. I can't understand how a ceased u-joint can shear these studs when the u-joints are non-serviceable other than replacement. I ask if u-joints are inspected as routine during "multi point" inspections but only get what feels like a dodging of the question. Can anyone say they have seen this failure before and what recourse I may follow. Thanks for any help.