It's a problem with just about any kind of car/truck; metal + extreme heat & moisture = corrosion.
If you have a nut & bolt set-up, I'd just grind-off the head or bolt and smack out the remains. It will save a lot of time and no innocent bystanders will get hurt. Wheel well is the best way, that I know, to get to it easiest.
If you have a threaded flange... then you need to get creative:
DO NOT USE a rust penetrating fluid on a situation like this!
Depending on your tool arsenal, comfort-level and knowledge, a torch, drill/tap & die could be a possible option. I would, personally weigh out that option against replacement.
I hope that this is some kind of help.
Good luck, none the less.