[FONT="]On my 01 XJ, the frame rail has either a welded nut or a nutsert that the bolts threads into.
Did it break off flush or is the enough of bolt left to grab with a set of vise grips?
Either way, start soaking them in penetrating oil and, using flat-tip punch, smack the end the remaining bolt(s), every so often to help loosen it.
Do this for a couple of days before proceeding.
If it broke off flush, try first center punching the bolt and then drill it out with series of drills, going from small to larger. You can try an easy-out but be gentle; it doesn't take much to break one off.
Good luck and stay cool[/FONT]