I have a 95 Jimmy and all of a sudden it won't reverse, I can still drive it but just can't back up. Do I need to replace the transmission or is it cheaper to fix the one that's already in it? Thanks for the help

Automatic or manual?
Fluid level low, linkage binding or disconnected, worn or damaged components, etc, can cause this "sudden" loss of a gear.
You should have a reputable transmission technician diagnose what has actually happened to your current transmission and discuss the options.