I dropped my pan for the first time in its life at 142k. Found the exact same thing. I believe its a spider gear/diff shim that got ejected.
Scared the **** out of me...but 28k later and the trans still shifts great. So much for that myth about not changing fluid if its too old....
Course, no clue how the POs treated the car, but I've got two giant coolers and an ATX temp gauge in the pan to keep the fluid temp in check. Yes I like to really beat on my ATX.