2000 Dakota overheating

I have a 4.7 Dakota quadcab that is overheating when pulling a trailer. The last time it tried to heat was pulling a 3800# trailer, well within weight limits. I have changed the thermostat, (no change), checked the water pump (it was ok), rodded and cleaned out the radiator (this helped, but it still tries to heat). Changed out the clutch fan. The gauge goes up when I acclerate up a hill, then I back off, and the gauge gradually drops back down.
What else do I need to try, besides shooting the truck?

We don't have the best radiators it seems. The Dakota forums have all complained about this problem in past at one time or another. Some step up to bigger radiators and have resolved the problem. They also added external transmission coolers.