I had this happen to my Pro Max sealed hub at the end of the winter. All of a sudden the nut just wouldn't move. I took it to the LBS, and they were as suprised as me (probably just thinking I was a weakling). They eventually got it off with the help of heat, a vice, and monsterous wrenches. After it came off, only a couple threads on the axel were done for. A new track nut threaded on fine, and I didn't have to replace the axle. I still have no idea what caused the nut to seize. I guess my advice is, take it to the bike shop.