doubt its anything to do with the ring gear

since he ruled out the speed sensor i would be checking the pressures from the pump since these units are mostly hydrostatic drive type systems

also check the linkage on the accelerator and the accelerator pot.
If correctly adjusted you should get these readings:
- Pedal full travel in forwards direction: 4.1V
- Pedal full travel in backwards direction: 0.9V

now depending on the model and year some of these units ran at only one engine rpm when drive is activated or the hydraulic levers were pulled if i remember correctly.
I would check the pedal position sensor and also the accelerator solenoid actuator.
Got a full model number to go with that serial number?