97 Bravada suddenly won't reverse

Hi, I've had this 97 Bravada for a couple months now, and suddenly after I drove it to school, got gas, and finally went home it would not go into reverse about an hour later when I tried to pull out of the driveway. It starts and runs fine, and as far as I know still drives great, but when I put it in reverse it barely moves at all and just makes this horrible clunking type sound if I give it gas. Any ideas? Thanks alot guys!

Here's a thread of a similiar problem,
http://www.automotiveforums.com/vbulletin/showthread.php?t=281604&highlight=sun+gear

Thanks you Chris, might anyone else have any similar experiences with a problem like this?