I've said it before, and I'll say it again. I believe the problem stems from overfilling the differential at the plant. Plain and simple. For those who don't know overfilling is worse than underfilling. When you overfill your engine with oil, overfill your transmission with trans fluid, or overfill your rear end with lube (get your minds out of the gutter!) guess what happens? I'll give ya two guesses, but the first one doesn't count.
Did you guess that it puts more stress on the seals, and the weakest seal gives way? Congratulations!
The thing is, though, that these differentials have a relief valve designed in. Maybe it's designed as more of a vent than a relief valve, I'm not sure. I've posted pictures of it elsewhere when I discovered that it was the source of my fluid leak. My differential was overfilled significantly, by the way. Regardless of what its purpose is, that is where some of my excess fluid escaped. I was lucky, my car didn't sh*t a pinion seal.
If the differentials are filled at the final assembly plant, then that could be the reason why this problem isn't prevalant on the CTS, to our knowledge.
Here's the scenario I picture... This may be a stretch, and it may not go down just like this, but here goes.
The new guy whose job it is to fill the differential with fluid (or programmed the dispenser to meter the correct volume of fluid) misunderstood the instructions. Maybe he was instructed that "full" is when the case is filled to the bottom of the threads of the fill plug in the casing. That is correct, by the way. In order to check to see if it is full, you jack the car up level, crawl underneath, and remove the fill plug. The fluid should be even with the bottom of the threads.
If the guy that is filling them does so at a bench before the part goes on the car, maybe he is just misinterpreting the directions. Maybe he is filling it up to the bottom of those threads with the differential
on it's side 
Or maybe I'm just off my rocker
