It is located inside the tailgate - you will have to remove the trim panel. Be careful not to break the tabs which hold the "Christmas tree" retainers to the panel!
You will also have to remove the rear wiper arm. In order to do that, you will have to lift the arm, put a small screwdriver under the little retaining spring that holds the arm to the wiper motor shaft.
Once the wiper arm is removed, you will have to remove the large, narrow nut that holds the trim piece (where the washer hose passes through the tailgate) in place.
While you're there, you may want to replace the trim ring, as they often become brittle with age, and the nipple which the washer hose attaches to will break. Many Jeep dealers still stock this item, and it goes for less than $10. Don't be surprised if your old one is gray and the new one is black!
Once you get those items removed, you will have to unplug the old motor and then remove four 10mm bolts from a bracket which holds the motor to the tailgate.