Spray the crap out of this nut with P.O. This is your axle retainer nut, and it is tight. Like 400 ft-lbs tight. Using an air impact (or a breaker bar and a cheater pipe) back it off. Flip the nut upside down and re-thread it backwards until the surface of the nut is flush with the end of the axle. You will see why in a bit.