Your mower has EZT trans and they have to be removed from the mower to change the oil. They are "sealed" and really do not need the oil changed. I changed mine because I buried them in a creel and they took on water. The most likely issue will be the drive belt. You should change it first and see if that fixes your problem. Make sure to take lots of pictures before you remove anything. This is very important. Link to your manual for the drives so you can change the oil. http://www.hydro-gear.com/service-and-repair-manuals/
Scroll down to ZT-2100, ZT-2200 (EZT) and download it.