Coming out of seclusion for this one. NO ONE knows why a diff fails, or really how big a problem they really are?
A differential always makes SOME noise. I am someone who was wiened on rear wheel drive cars. There is a difference between fWD and RWD autos obviously, and I have a theory that with the CTS there are some people who buy this car and its literally their FIRST RWD car. Therefore they hear rear wheel whine and say WTF is that?
NOW, obviously a failed rear differential would make all kind of noises before it would seize up, or even possibly explode under load. How many of these supposed "failures" of a CTS differential is an actual failure and how many is just rear differential whine is anybody's guess?
There is no REAL data that tells us how many of these there are, what is causing it, and how big and how REAL a problem this is. Cadillac would be able to answer this question, but for obvious reasons they will keep it close to the vest. My guess is this, a differential failue would be a pretty serious occurance, serious enough to warrent a recall by GM, and we have't see that as yet, so my gess its NOT that big a deal or just not as serious as all that.
If GM ever does a recall, that would alert you to some manufacturing or design defect.