Is it just one of them or all of them? I'll bet you could get a good angle from underneath or through the side of the bumper with the opening. Maybe vicegrips?
This link has some info that might help.
http://www.fordraptorforum.com/f170/have-you-stripped-your-lower-skid-plate-bolts-32604/
Just the one...i stopped after the first one broke. Didn't want to break the others. Gonna try and get a wrench in there from the side. Should work.