Your tractor has a Tuff-Torq K46 transmission. According to Sears they are supposedly sealed and non-repairable. Only unit replacement is offered by Sears.

Your symptoms sound pretty consistent with a transmission that is failing. While a long term fix is to either replace the transmission ($600-$700) or rebuild it ($350+your time), you might be able to get some additional miles out of the unit by changing the oil in the transmission.

Originally the K46 was filled with 10W-30 motor oil. Tuff-Torq recommends replacing with 10W-50 synthetic. I have also heard some folks have had even better success with the Castrol 10W-60 that BMW specs for their M3-M5. However that oil is about $20/liter, and you'll need 2 or 3 liters.

In any case, changing the oil isn't a long term fix. Your pump is worn, and the heavier oil only serves to reduce clearances and improve hydrostatic pressure.

Thanks ggoyeneche for the quick response. I did see in the owners manual that the trans is sealed. How then would I go about draining the old oil and replacing with synthetic?

https://www.google.com/#q=tuff+torq+k46+service+manual

A good first thing to check would be the integrity of the belt.