I like your come-a-long and tree idea. What I'd do is leave two nuts on, threaded on only a few threads each. That way, when it pops loose, the whole assembly wont go flying.
After you attach the come-a-long, put as much tension as you dare on it, and then smack the axle housing hard with a good sized sledge. Your air hammer likely doesn't have enough shock value to it.
If it still doesn't come after doing that, find an acetylene torch with a rosebud and heat that area up. Then, it's beat and heat until it comes loose.