Two nuts. I had the same problem with my XJ. I'm assuming you twisted the head off of the bolt.
Screw the first nut on to the part of the bolt that is sticking out under the Jeep. Put the 2nd nut snugged up to the 1st one. Turn the 1st one with a wrench. The 2nd nut will keep the 1st nut from unscrewing.
Jeeping - The art of getting dirty & going broke while slowly heading nowhere, and taking all day to do it.
97 XJ SE - DD/Light Trail Rig - The War Wagon
75 CJ5 with a badly chopped CJ7 Tub - Pandora
96 Land Rover Discovery - Black Dog; possible trail toy