I recently took my 99 XJ on a 3 day excursion through Death Valley. Went up and down 3 different mountain passes, all with varying degrees of difficulty, out to Saline Valley hot springs and back to cabin for a total of about 150 miles offroad. It ran fine all throughout that, its when I got to about 3/4's up the access road to the cabin. Coming up a 40 degree incline, it lost forward momentum, would not move but the engine was on and working, then it started to overheat. I shut it down and let it sit overnight on the hill. I started it back up in the morning and made it about 20 more feet up the hill before the same thing happened. I turned around and parked it, got my stuff from the cabin, went back to the XJ and started it up, drove it to the base of the hill. It drove fine downhill, no issues. When I got ready to drive, the temperature pegged. I moved a heater hose (was being pinched by hood) and refilled the radiator with antifreeze, bled and drove the 150 miles home. Once there I changed the transmission fluid, which was brown. No burnt smell.

My question is: What caused the XJ to not make it up that hill? It had made it up the same hill the night before. I'm thinking it was the transmission fluid?

 

I assume you would have changed all the fluids before this excursion, so if the transmission fluid is brown (suspended friction material) this quick, the transmission needs some attention.