Locks up when put in reverse

I got back from a day trip, parked, turned off my 2007, but when I got back in an hour later the car would only go a few inches in reverse, then it jerks to a halt as if I had suddenly mashed the park button. I engaged the parking brake, disengaged (light going on and off), but still locking tight and not budging. After a few tries -- each time only going a few inches backward before the "park" engaged, I put it in neutral and pushed it out of my face-in parking place, then put it in drive . . . and it does go forward, but with a grinding sound. BTW, on the way home (100-mile trip) I did hear a clang-bang underneath as if a tennis ball-sized rock hit the undercarriage while cruising at 60 mph, but nothing seemed to be wrong, got home fine. But then the no backing up started. . . What could this be?

 

Thanks folks. Actually, the neutral rolling wasn't easy either. I'll get it to my mechanic (hopefully) and he can look at it. Sounds like there's no "a-ha! I know what it is!" answer then. . . .

 

Just got it to my mechanic. On the way there it made grinding noises with a typical wheel periodicity, i.e., the faster it turned, the more frequent to grinding sound. My mechanic took one look and said it was the front left bearings. Logical, but still confusing about the reverse lock-up. Let's call it a day until mechanic takes it up on Monday.

 

We're fairly confident, since he replaced the front-right bearings just a month ago. I was waiting for the other shoe to drop. . . .