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I've been trying heat-cycling instead of just temp differences when I get stuck. Heat, apply ice. Wait till ice is melted. Repeat several times.
If you're ready to call the bearings and maybe the axle casualties of war, put the nut back on 75% of the way with a little threadlocker. Once threadlocker dries, start wailing on it with a sledge. Shock-and-awe approach also likely to bust up corrosion. Might need a helper to brace the bike to make sure you're getting the most out of the impacts.
If this thing has been apart within the past year, hard to believe it's sledge-hammer stuck.