There are many videos on YouTube covering replacing/servicing/repairing hydraulic transaxles. The whole key is making sure you get the correct replacement before removing and replacing.
give us the actual model from the serial tag and we might be to able to find the service and parts list. I for one hasn't had one these in the shop yet.
EDIT > I manage to find a service manual and it is a joke of a service manual.
I think this model of Hustler uses pumps and wheel motors.
You will need to provide the model number and serial number of the mower.
You may need the serial number and model number of the pumps and wheel motors.