Welcome Oak glad your here,
Does the drive shaft seem to vibrate?? how do the flex discs look? I belive if you remove the driveshaft and the coupler for the drive shaft on the input shaft of the charge pump you can pull the seal and replace it.
Clean the heck out of it,then run it again and check for leaks,trunion seals/the linkage that controls your forward and reverse movment,and where the whole unit meets the rear end.
If the trunion seals leak you will have to break the whole unit down/the hydro,and if the gasket that is between the hydro itself and the rear,its easier to split the tractor to replace the gasket.
Check the damper spring plate to see if the springs are wore.
I looked up your tractor on the parts look up and it looks like there were two different hydros depending on serial # ????
Here is a link
http://cubcadet.arinet.com/scripts/E...&loginpwd=test just type in your model and it will give you an illastrated parts list.
If you chose to take the hydro apart keep it extremly clean!!! Your gonna have to clean up everything really well,work the tractor a bit and then check for all the leaks
Good luck,Jason