Engine cooling.

check to make sure that the fan is coming on a coolant flush will not fix this and I would not recommend any additives to the coolant system to make it colder run the truck for awhile see if the fan comes on if it does not apply 12 volt power to the fan if it comes on then check that the wires are not torn or broken then check the relay if it does not come on then look for signs of a faulty fan burnt motor etc

it might help. considering the truck runs properly when not towing it sounds like that could potentially be the problem. another problem i could think of is maybe your thermostat isnt opening up properly or your fan isnt turning on properly either. try the flush. it couldnt hurt and probably is a good idea if you have a bunch of miles on your ride now.

one of the biggest problem i had with my 1996 cummins was overheating while towing as well. what i did was change over my fan to electric and then change my coolant protection from -34 50/50 (antifreeze/water) to -28 40/60 (antifreeze/water). i had a friend of mine help me with this decision as he did the same, after that i didn't have a problem.