My buddy has an 01 thats all stock except for a straight pipe and hes having problems with it overheating. But its only when hes pulling a trailer and it goes all the way to the red, unloaded its fine. So does his truck have a thermostat that could be out or is it the waterpump.

 

check the radiator, it might be plugged externally,
if it still has the "puke bottle", oil vapors will attach to the radiator and stop the air flow
https://www.dieseltruckresource.com/...ht=puke+bottle

 

My 2000 3500 dually was doing the same. I was advised by a diesel resource thread and I pulled the radiator and found a lot of stuff plugging up the outside of the radiator. I actually took it to a radiator shop. They said the inside was fine. They did clean it and even rodded it out but they told me that the only thing they could see was the outside was oily and plugged up. I put in a new "Cummins" 180 degree thermostat and have pulled some pretty good hills with my 9000 pound fifth wheel with the heat guage going
up to about 200 degrees and no more.

 

i gotta clean mine. i keep putting it off.

 

I had the same problem, radiator was plugged externally. I had to pull mine and get it cleaned at a shop, garden hose wouldn't do it.

 

I had the same problem. After a good degreasing and relocating the bottle, it doesn't even get to hot pulling my toy hauler (15000 lbs) in 110 deg weather.