If 5 bolts are spinning the welded nut on the inside of the frame broke. If there is some room between the bottom frame and the skid plate upper lip you can cut the bolt, maybe enough room for a hacksaw or saws-all. That will drop them out. Then you're going to have to "re-weld" the nuts back on the frame.
Getting this done correctly and safely is priority, put your SYE on hold for now. I've seen pics of guys who have done this wrong and the trans plate broke while driving bc of the weight, you can imagine the damage it caused. If you do the threaded stem option mentioned above, make sure you only use grade 8 hardware, nothing less.
Location: East of Gratiot, West of Zanesville, Ohio
I did what 95YamJam did. I used a grinder to make slices across the nuts that wer in the frame. Then I drove them out with a hammer. This created an indentation in the frame. Then I made plates of 1/8" steel with holes in the center. To those plates I welded heavy flange nuts. Then I drove those nuts into the dents in the frame until the plates bottomed out against the frame. Finally I welded around the edge of the plates, ground the welds out, and painted.
Great options guys, going to check tractor supply for class 8 hardware to go through and through, the welding deff a better option, but mine is an offroad YJ only, if I can't find the appropiate hardware it will be welding time.