I am still sticking to the idea that the clutch is bad or going bad. Mine appeared to work just fine, but under any torque it was slipping, and my temp would get hot, even when driving.
As for speed, the RPMs are more important than actual speed. The lower the RPMs, the less combustion, therefore less heat generated. I would NOT run your AC, as a matter of fact, running the heat on full will help reduce the engine heat, but as long as it isn't in the red, you should be ok. You can also crack the hood, cut a hole in the hoodI would change the oil after this problem is solved, if it has run hot a little bit, your oil will start to break down. No worries, don't drive it in the red, get it home, should be no problems, I have over heated mine a few times on the trail. Another trick the ZU guys showed me, and this will determine if its the fan, pop the hood, with the truck in park, rev the engine to about 2-3k rpms manually at the throttle body. If the fan isn't sucking a bunch of dirt out of the radiator, your fan isn't working, mine is like a hurricane under the hood when I do that and you can drop temp back to 170 in less than a minute.