1 of 2 things happend with the nuts in the frame when the PO had it. The bolts broke when trying to remove them, or the threads were so bad the bolts wouldn't hold. Retapping the nuts with the same size tap won't help if its the latter. The next size up tap could be used to cut new threads.
I saw the same thing happen with a '97 TJ with 137,xxx miles on it. The bottom of the frame was rotten.
1957 WILLYS pickup,
1973 J 4000,
1978 CJ7 DD.
1979 CJ7 360, TH400/Quadratrac.
1980 CJ5 trail Jeep.
1989 YJ the CJ to YJ.