Safety first, use jack stands and remove both rear wheels. This should be done at the factory but is not. Remove rust from axle shaft with a scotch brite pad and apply anti-seize compound to axle shaft with a small paint brush liberally. This might not resolve your problem but will ensure rear wheels will not seize to axle shafts.
Check oil in your oil cup first, if low add 30w just 1/4" over the bottom. It could be the couplings between the pump and trans. The shafts have very fine splines, the couplings also have splines that go bad and will need replaced. You must removed the rear end from tractor to removed the pump to replace. There will be four couplings, replace them all. You will not lose any oil.
