PCoD

Recently had unusual noise mostly around 50 mph. took in for inspection and told u-joint had back of driveshaft failed and sheared off two of four studs to the rear differential coupling resulting in a $3100 bill. Has anyone heard of this before (dealer says they never have) and possibly explain how it could happen? U-joints are non-serviceable sealed units so if they are subject to failure I would expect that be part of some periodical service inspection.Car was in for differential service 9,000 miles ago and currently sits at just over 121k. I would think this should have been caught during differential servicing. Any thoughts and advice is appreciated.