Remote will not unlock my back passenger door

You'll have to remove the interior door panel. I'm not familiar with Cherokees specifically, but most door panels are held on by 20 or so clips all around the panel. First look for any screws or bolt heads. These may be under the arm rest or covered by plastic covers. There will probably be some around the console. Remove all the screws and bolts you can find. Then start at the lower corner of the door with a small flat object, like a scraper or small prybar. Insert the tool between the steel of the door and the door panel. You should be able to pop some of the clips, loosening the panel. Work slowly around the bottom and sides of the panel. There will be some clips in the center that you'll have to pop with your hand. When you have the whole panel loose, you'll need to push it up to get it out of the window groove.

Be careful as the panel comes off, as it will have wires connecting to the window and door buttons. Detach these wires and set the door panel aside. Now you should be able to see the motor that activates the lock. When you push a lock button on the driver's door, you should be able to hear the noise from the motor. See if it is working correctly and has come loose from the lock, or if the motor itself is not working. There are probably plastic gears in the motor that can strip, which means you should replace the motor.

Hope this helps. I know it's a little vague, but you should be able to get in there and see what's happening.