If it was in the center of the rear frame crossmember, it was probably to hold the spare tire metal bracket. On civi jeeps the bolt looks like a lug nut but with the long threaded shank instead of a hole for a lug stud. You would loosen the bolt and the spare tire bracket has a slot with a stamped pocket you can slide off the lug bolt and lower the tire without removing the bolt all the way. That bolt looks to be roughly the same length.