I have three customers with that exact model of tractor. The axles inside the wheel hubs are double D shaped and are prone to breaking off just inside of the wheel hub. Jack up the rear of the mower and try to turn while pulling outward on the wheel and see if it falls off the end of the axle. Also the cast wheel hubs will also strip allowing the hub to turn on the end of the axle like the keyway is missing, but that will give an appearance like the top of the wheel is tilting inward.