OK, I'll bore you with another vote for heat but...use an acetylene/oxygen torch and heat the nut to a dull red. Then throw the impact wrench on it and the nut will spin right off.
Heating those nuts will not damage anything. The seals are far enough away as others have mentioned. However, the area, nuts, and bolts will stay very hot for a long time. Best let them cool down on their own. Have lunch, a few beers, or whatever for an hour or so.
On the impact wrench...some of the cheaper wrenches, even in 3/4" or 1", won't do the job like a 1/2" quality (read $$$) impact wrench. Been there, done that.
Edit: And when you put the nuts back on just let the impact wrench rattle on them for a couple of minutes. The torque will be perfect then. :thumbsup: