Overheating on 2000 4.0L

Truck's a 4x4 5sp off-road w/ 48k, pretty sure everything is stock. Running on the highway will make it run hot. Running with the A/C on will make it run hot. Doing both of these will make it quickly approach overheating. Then today I was taking a light load 5 miles and it was about pegged, I had to turn the heat on full blast to make it home. Where the frick do I begin in fixing this?

uh, i would start by checking your hoses, and being sure that coolant is circulating. (pull off the inlet house while running cold) if not that, i would look and see if you have a clogged radiator somewhere.

Replace the thermostat!! should fix the overheating, if that is the problem.

$ 2.00 thermostat.

well i mean, yeah if thats the problem.. but you can test the thermostat by boiling it, and if it opens at the appropiate temp its fine, but you should check for circulation as i described first.

Thermostat is a good place to start, but with it heating up on the highway, I would look at your radiator, you could have flow, and still have a clogged radiator. Also, is there coolant dripping from the weep hole in the bottom of your water pump?

I've had the same problem twice. The first time I replaced the thermostat and it was fine. The second time was a couple months ago. Come to find out one of the hoses was kinked so there was no circulation. I would definately look into the t-stat and check your hoses.