Truck overheating towing on long steep grades.

That series of CTD trucks had a known defect that made those symptoms occur. At the rear bottom of the radiator "fuzz' will build up and clog the fins. Clean out the fuzz on a regular basis. It happened on my 2001 CTD at 75,000 miles. After that I drove it for another 250,000 with a mishap.
Dodge told me I needed both a radiator and transmission ($3,000). I laughed and searched the CTD forums.