Remove the grille, slit the back of the enclosed plinth bit to remove the old floating plate.
Treat any rot and touch up with weld through primer.
Grind the old nut off the floating plate and replace with new full plain nut (3/8 UNC I think), paint with hammerite, INCLUDING the threads, replace nut plate in plinth.
Weld access hole closed and paint to match Jeep or local mud colour.
Reassemble and admire.
Alternatively you could remove the floating nut as above and either helicoil it or drill and tap to a larger size, (possibly metric). The advantage of a helicoil is that it won't rust, not to say the bolt won't though.