Blazer Tranny?

Mother-In-Law's 2002 Blazer won't go in reverse. It's parked in the garage right now and she can't get it out without pushing it. I checked the tranny fluid and it's nice and pink and full. I can put it in drive but it won't go in reverse, just sits there and revs. I put it in 4wd (push button on dash) and it does the same thing. She said the other day she came around a corner and she saw the RPMs go up but it seemed like she wasn't going any faster. Is there different linkage in the tranny for drive and reverse? She bought it used and naturally the warrenty is up! Will she need a new tranny or get this one rebuilt?