ez-out isn't gonna help get a whole bolt out, and unfortunately a spare caliper won't do you any good either.
What you need is a pair of vise grips like machinisttx said, grab the head and twist back and forth while pulling back. Once it's out, you will need an M8x1.25mm helicoil kit, most good parts stores will have one, I know advance auto had one in my area. Bore the mounting hole on the knuckle out with the proper size drill, tap it with the included helicoil tap, insert the helicoil with the tool, and reassemble everything.
Almost any parts store can get you a new bolt, they're M8x1.25 but I'm not sure what length. Just ask for front caliper bolts for whatever year XJ you have.
Speaking of which - what year is your XJ? The above advise holds for ~92.5 and later, earlier ones are a dizzying array of different designs depending on whether they are 4wd or 2wd and what specific year they are.