Hello....
Our 1999 Dodge Diesel (2500), likes to overheat while climbing, towing uphill, and on hot days, (above 80 degrees F).
I replaced the thermostat wint one 188 degrees, from Carquest,
part no. 32809, $32.00.....Before the temperature averaged 170 degrees, now it stays at 190 degrees, as per the gauge.
The fan clutch appears ok, according to what I have read and tried.
The radiator fins may be plugged, hard to see because of the ac
cooling radiator, I have use gunk twice, trying to clean it as good
as I can see and do. The temperature while climbing on a hot dirty day, goes up to 230 degrees, so I suspect that if I tow it will boil
over. Any hints out there?

 

Two thoughts popped into my head. Maybe the inside of the rad is plugged up and it may be time to get it rodded out.
Second, how about a bad water pump? The impeller could be badly eroded or coated.
Remember that if you have an overheat condition as an emergency to slightly lower the engine temps, place the cab heat on high and the fan on high. Open you windows and sweat.