MTD Lawn tractor 1994 model 134M679G062. Won't move forward or reverse. Thought it was a broken belt, but both front and rear drive belts are intact. Seems like maybe a spring came loose or broke??

If your trouble was the same as mine... spray some WD40 on the Variable Pulley (the one that has both drive belts on it). The pulley has a disk that's meant to slide up and down depending upon the speed. Mine was dirty and stuck and prevented the belts operating as designed. WD-40, and bit of time running to let it soak in and I was back up and running.