Have you tried soaking it in penetrating oil? Spray Liquid Wrench or something similar on it every few hours for a day or two. Then, try to work it back and forth. If it binds up again while working it out, soak it some more and repeat. I have a bottle of this stuff called "Kroil" (I think). I got it from a machinist here at work. Smells like wintergreen (which is similar to some stuff our airframers used to use on military jets). The container says, "The oil that creeps!" It does work very well. Back the locking nut off, and soak it down for as long as you can. Be patient, and it might finally work out. After that, I would run a tap through the threads in the swingarm (6mm-I think-maybe 8mm) to clean them out. If the bolts' threads are corroded, wire brush them, then shoot a dash of some oil on the bolt and swingarm threads to help prevent this from happening again. I usually spray mine down every time I adjust the chain or take the rear wheel off any bike. Good luck.