Hi All - I have a 2001 grand prix GT that has developed a "clunk" in the transmission. Any thoughts on what it could be or what to look for? Differential, filters, other?

2001 Grand Prix GT with ~ 105K miles.
Clunking sound is observed when shifting from park to drive, park to reverse, or reverse to drive. Clunking is also sometimes observed when accelerating (transmission is not shifting when sound is observed during acceleration).
- I've checked the engine and tranny mounts
- Tranny fluid is at proper level
- It does not appear to be anything in the suspension or CV joints
- I crawled under the car to observe the sound, and it appears to be coming from roughly the center of the transmission (left/right)
- The noise showed up after I got stuck in the snow a couple times. While getting un-stuck, I removed traction control and spun the wheels quite a bit.
- I've been told that the differential can be damaged when doing this and it will exhibit a clunking sound, but will work until a stake works it's way out of the shaft, at which point the car will no longer move
- Has anyone else heard of this or have any additional details?