Transaxle problem-mower barely moves and then stops altogether.

I have a Craftsman GTV 16 garden tractor with Varidrive. 16 hp, I believe it's an early 80's model. There is a model and SN plate on the front frame (917-254 050), another one on the rear frame (917-253 721) and another one on the 3 point hitch, none of which match. So I don't know which one would apply to the transaxle. The previous owner gave me a Sears Owners Manual that seems to match the mower with yet another model number (917-253 724). For what it's worth, the part number for the transaxle assembly in the owners manual is 633A110. When I put it in any gear (standard H pattern) it just creeps along, barely moving for a few feet and then stops. I can also see that the pulley on the side of the transaxle that drives the belt to, I suppose the mower deck, (I haven't mounted it yet), is spinning until the tractor comes to a stop and then it also stops spinning. Before ever starting it, I checked the fluid level in the transaxle and it was full with what appears to be fresh oil. Does this sound like a transaxle rebuild or is there something simpler that I could try first?

Thanks for any help!