98 Explorer. Reverse Went Out!! Please Help!

I have a 98 Explorer XLT 4x2 4.0L 4 Door auto tranny. The other day my reverse stopped working! It has 120K and all forward gears seem to work just fine! Am I looking at a large repair bill? What could be wrong? Please Help! I'm car savvy but don't know much about trannys. Thanks! Coops Dad!

Sorry, I have to tell you this. your going to need it rebuilt and will be costly. I had my reverse go out on my auto tranny it cost $2,088 to have it rebuilt and a new sensor put on. What happened was one day started it up in the garge put it in reverse but it acted like it was in neutral. Tried a couple more times but with no movement pushed it out put it in drive and shifted fine drove around the block and parked it. Called a local mechanic had it fixed runs fine know. This fix is not smoething you would want to do yourself. It took that mechanic a week to have it ready to drive and my truck was the only one being worked on.