tremblay

I have a Craftsman 19hp Twin Turbo lawn tractor with hydrostatic transmission. Recently, the tractor does not go up hills or at any incline like it used to. When the tractor first starts, it seems to be OK. Then, as it heats up, a high pitched whine emanates from the tractor when I attempt to go uphill. I changed the front motion drive belt, but this had no effect. Sears will charge me $96 just to look at it, and if it is the transaxle, it would probably be better if I bought a new tractor?? Looking for advice...it is a sealed unit, but is it missing some fluid? Thanks for any help.