I didn't see if he bought the Xj in this condition. If he did, it's possible that all 4 motors are bad. there are some people out there use it till it breaks and than just quit using it instead of repairing.
When you jumped it with a hot lead did you also jumper in a good ground?

 

The ground would be the only thing I would worry about. I believe the window motors are a chassis ground but I could be way wrong. He did say (if I remember correctly) that they did meter things so I would hope they metered the power wire he ran.

When he ran the wire directly to the motor, he essentially bypassed anything in the system that could be bad, (switch, relay, fuse, yadda yadda). Like I said before, try and remove the (motor) from the (regulator) and retest the motor. You may have something as simple as a dirty regulator or a weak motor. Jeeps have an inherent issue with the regulators getting dirty and causeing issues like this. If this is not the issue you have a bigger problem. If all 4 motors went out simutaniously then that would leave me to believe you had some sort of a fault or an excesive current draw causing other things to become damaged.

If you would like you can PM me and Ill give you my number so we can have a conversation about this.

 

Windows motors get ground from the driver's door and they run under the carpet. Mine were broken and yellow. After fixing it all windows work. They used to only work from the driver's door switches.

 

i have a 1993 jeep grand cherokee laredo all windows worked at one point and after a while only the drivers master switch worked and now that does not work but the driver door lock does work. I took it to my mech. he disconnected the master switch from the wiring harneses and checked for power in each harness conector and there was power. He tells me the problem is the switch. I went to a auto salvage store they got me a switch I connected it and that switch is not working either. someone please help it's hot in NYC.