The model number you've entered says you have a 520-H, 1993-1996 model.
When I have the rear wheels jacked off the ground they spin freely without any resistance
I'm curious, based on how you said this. With the wheels jacked up, one wheel should turn in one direction, and the other should turn the opposite. This only proves the differential is OK, because you aren't actually making the whole transaxle move. You're simply engaging the diff... Can the tractor be rolled around freely (pushed or towed) without binding up? If not, DO NOT FORCE IT!!!!! You may have a differential bolt or something similar that has loosened up, and may be contacting the inside wall of the transaxle. Your transmission is completely capable of destroying itself if you put it in a bind, simply because of the torque it can generate. Don't put too much pressure on it.
The good thing about the Eaton tranny on your 520, is that you can remove one side of the tranny case without taking the whole tractor apart. A visual inspection of the inside of the tranny would not be that difficult if you're confortable working on your own equipment. I have been inside several Wheel Horse hydros, and the Eatons are not very complicated. The most important thing when opening ANY of them is to keep ANY trash out of them. The smallest amount of grit in the pump will destroy it, and the pump is where you get into the high dollar repairs, if you can even find the parts.
My suggestion would be to verify you can roll the tractor around without it binding with you. If not, your trouble is in the transaxle, not the pump. That would be a good thing.