The square piece that the nut sits in is actually part of the body. In order to access the nut, you need to cut a hole in the floor. Pull the other half of the bolt out, install the new bolt, and weld a patch back over the hole.
'87 YJ, 4.2L built and bored, 8.5" of lift, 8.8 rear, AX15/NP 231, Tom Woods drive shafts, Flat fenders, 12,000LB winch, Custom bumpers, Rattle can paint job. Never completed...