Passenger door swings closed on its own...

Most vehicles (the Wrangler is an exception) have some sort of spring gizmo that tends to hold the door open when open and closed when closed. I think yours is part of one of the door hinges, but someone who actually owns an XJ and has it close by can answer better. Take a good look at the hinges on the side that operates properly, and compare this with the side that does not. I think you will find that something is broken or missing on the side that does not work.

Earle

If you look between the hinges, you will see a flat bar attached to the pillar with a pin. That serves to limit the travel so that the door doesn't open too far and also acts as a catch to hold it open. Inside the door, the bar has a notch that rides on a spring loaded roller (part of the door side bracket). Every one I've got has broken the bracket ear that holds the roller and spring so the bar acts as a limiter but the catch to hold the door open is busted loose and doesn't work anymore. I just put up with it - it's a PITA to remove and re-weld when it's just going to break somewhere else shortly anyway.

It's called a door check. It's a spring loaded detent. Many times it's just a matter of the pin breaking or falling out. It's located between the hinges if I remember correctly.

As Earle says. Open both front doors and compare. It'll probably be obvious.

there are suppose to straps to hook to keep this from happening is there not there maybe who owed the jeep before you cut them off.