They fail in a few ways. The bearings wear down, and the races wear out, along with the carrier (or whatever it is called). Everything should be in there pretty tight. I changed mine out, I only found one race with some scoring, but both shafts were clicking and clacking like crazy every time the suspension would shift a bit.
Changed them both, packed the new ones with Mobil 1 synthetic red grease, and they were whisper quiet. Didn't even get 1000km's on them before the car burned
