Changing shakles on a '96 XJ

I tried to add lift shackles on my '96 XJ yesterday and the top bolt holding the shackle on seemed to thread out of a nut that seemed to be welded to the frame. After about 4 turns the nut broke loose and is now turning with the bolt. Since the nut is in a recessed area in the box of the frame I could not get to it without cutting part of the rear of the box. Even then I can't get a socket or a wrench on the nut. My final attempt was to zaw-zaw the bolt between the shackle and frame but it's too tight to get the blade in there and I'd bet the bolt is a grade 8 or tougher. Any suggestions on how to get this bolt and nut out?

Thanks,

Brian

Rolled it? Flip it back over and keep on wheelin!

cut a hole in the floor above the nut. then patch it up when you are done