If you don't want to risk damaging the shaft with the vice grips you might want to use an adjustable wrench to hold the top of shaft (right above the nut) and use an open ended wrench to turn the nut until it's loose. Hopefully by the time you run out of room to use the adjustable wrench(to hold the shaft still), the bolt is loose and not stuck on messed up threads or else you will really have to use the vice grips.
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-Juan
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