I'll throw an off the wall thought out there. Since it seems to happen to some but not all, and many seem to lean towards the differential, here's something to maybe look into. I don't know much at all about how the E-diff works or how fluid level affects it. But GM has been known in the past to not completely fill the differential. I personally have seen it only about half full on a new vehicle but it was years ago. I would hope that is not possible today on a car like this. But just to rule it out, maybe someone who is having this problem could check differential fluid level? Again, I know it may be a stupid thought but just brain storming a bit.