What is best way to remove rusted rear shock top nut?

Just cut the top off, or on the shock shaft itself, grind two sides flat so you can get your vise grips on it. then that'll keep it from spinning while you undo the nut. Either one works just fine.

 

change it sooner next time..........

 

I guess two years and only about 5000 miles was to long for mine. I will remove the 1,000 mile 6 month old foxes tomorrow and sell them. Just kidding they are too short so I am removing them for a different reason

 

Thanks for all the replies. Some useful, some not. I used a sawzall and it was 10 times easier than a cutoff wheel. New shocks installed. Here is a picture of the old shock and cutoff nut.