Hello TOC.
Sorry for the late reply. I feel bad for missing your response so even though it's very late, it's bad etiquette to leave this thread open.
My daughter's Matrix was the troubled vehicle. The mechanic she went to said that the parking pawl was jammed. I wasn't there but she told me that he did something that sounded very unconventional but apparently it worked. He put the car on a slight incline, while in reverse (I think), let it roll very slowly and then without breaking, threw into park. That unjammed the pawl and solved the problem... for now. He said that it might only be temporary and if happens again, she might be looking at dropping several $k for a new tranny. So far, no more issues. She admitted that she is guilty of both not using the P-brake and not waiting for the car to come to a complete stop before shifting to park... and there's the cause. Obviously, I told her that unless she has $3-6k of disposable income laying around, she'd better knock it off. Message received.
Hope this thread proves useful to someone. Again, sorry for leaving you hangin'.