edit. brain no worky.

take the axle out and use vice grips or other locking pliers to grip the center portion (non-threaded) of the axle or stick two pieces of scrap wood on each side of it and put it in a vice. If you get the nut off and the threads are munged up you can sometimes clear the threads by running a nut up and down on it several times. Then again, it's prolly easier to just get a new axle