Can I conclude that there is no overheating problem once the truck is up to speed? As @Barkydog suggested there may be an accumulation of debris in the condenser and also the radiator that the fan cannot overcome. If the radiator were internally plugged the overheating would be worse at highway speed in warm weather.
How long have you owned the truck? If the truck is new to you how do you know the problem has been around since mile one? And if so adding an electric fan at the front of the condenser to boost the air flow when needed might be necessary. If you are desperately needing the AC in March you’re in a hot region and an additional fan is often needed in extreme heat. And the dual evaporator AC does add a great deal more heat to be dealt with by the cooling system.