Ok so i have a mildly molested 90 cherokee and when the headlights are on the brake lights stop working. I checked all the fuses and they are fine. when the head lights are not on everything works just fine. I need to make a 400 mile drive in less than a week. So any help would be very helpful.

 

check all of your grounds. Add a new one if you need to,also if you have trailer wiringcheck that for corrosion

 

Not sure if XJ tail lights are similar to WJ tail lights, but check the contacts. I had a similar issue with my WJ, and turned out part of the bulb was not making contact with the tail light itself.