Alan, Welcome to the Forum. You'll find a lot of friendly and knowledgeable Cub lovers here. If you click on "PDF manuals", or "Rudi's Manuals" in the tool bar above, you will find Owner's, Operator's, and Service Manuals for your Cub. To answer your question, the gear oil for the bull gears is held in the pans underneath the rear finals:
Picture from TM Tractor. To change the oil, you need to remove about 10 bolts and drop the pan. There is a cork gasket there that will probably need to be replaced. I'd suggest changing all the fluids as shown under maintenance in the Owner's Manual, unless you know the previous owner did it regularly. Any other questions, feel free to ask.