So that last post was back in March of '09. Today is 12/1/09.
After that last dealer trip back in March, the leaking stopped and the car seemed ok. Over the summer, I started to notice some occasional grinding/dragging while going around corners. Especially when warmed up. I took it in to the dealer to ask them to check the differential. They dismissed it as normal.
In October (around 30k miles) the grinding got much worse. Bad enough that you could really hear it even from outside the car. It would grind on every corner, regardless of speed, temperature, or sharpness of the turn. I took it back to the dealer and told them that there's no way this is normal. They agreed and said there was a TSB involving a different fluid additive. The grinding went away for about a month. A few weeks ago, the grinding suddenly came back, as loud and obnoxious as ever.
I took it right back in. This time the dealer agreed that the clutch packs are shot and GM told them to replace the whole rear end this time. Nice. It's getting that done today. Glad this is all under warranty, as the dealer says it'd be about $3,000 worth of work if it weren't.
So if anyone is reading this with an '07 GXP that's grinding around corners, TAKE IT IN!!!