Need some input on what you guys think it is before I tear into it. 2003 Silverado SS AWD 6.0. Had it parked in a parking lot, went to leave put it in reverse and it clunked really hard and next thing i hear is grinding. Put it in Drive, same thing. Will not go anywhere just grinds really bad. Lost park as well, you can push it around in park. When I towed in to my barn just towing it in nuetral I could hear the grinding with the truck off. Any ideas? Tcase? Weak points? Thanks