Help! Truck won't go in reverse! Neutral is drive!

If anyone can help us it would be so very appreciated. We own a septic service and have a 1994 Peterbilt with a CAT 3306 and Allison automatic (mechanical) tranny. The truck suddenly doesn't seem to know what gear you have it in. It won't go at all in reverse, and it drives in neutral. We checked the fluid and it's fine, then pulled linkage out and tried to pull it down into reverse manually and it still won't go in reverse. Any help would be wonderful. This is our only truck and we are up a creek without it.

Thank you.