Another thing to check while you are getting this info. is to check up underneather the tractor on the right side (as you are sitting on it).
There are a series of solenoids that are positioned on the transmission case that have wires going to them. Are any of the wires broken or torn off?
This is problem on these tractors especially when working in heavy brush and vines. They can become entangled under the tractor and tear the wires loose. The 4WD engagement solenoid is typically the prime candidate this happens to.
Sometimes you can solder repair the wire if they are not pulled out from inside the solenoid and if they are, you will have to replace it.
Be sure to let us know how you make out.