That's overkill for a 1,500# trailer. I wouldn't do anything.
How is the additional tranny cooler installed? Ford put in an auxiliary one at the factory, in addition to the one built into the radiator. Did you replace the auxiliary cooler?
The cooler in the radiator is very efficient--liquid to liquid. Under most circumstances it makes it impossible to cook the fluid because it will strip the heat rapidly out of the tranny fluid down to the engine coolant temperature. The air to liquid cooler after it covers extreme conditions and you shouldn't need anything more than that.
I would not take the radiator mounted cooler out of the circuit. In the chart I inserted below from TCI, you can see that the engine coolant does not heat the fluid up harmfully. Using only a liquid-air is dangerous because it is not nearly as efficient. Think about how long a welded piece takes to cool waving it around in the air compared to dropping it in a bucket of water. It's the difference between forever and instantly.
The best thing to do is leave the truck alone, it's already been set up by Ford to tow probably around 6,000# with your 4.0. Heat in the drivetrain is controlled by how much torque you are asking the engine to produce. A lower rear axle gear is the best thing. For instance, 4.10 gears mean the engine has to produce 33% less torque. It spins faster, which cycles the coolant faster, and has less heat from the cylinders to take away and more coolant to do it with. On grades, keep the motor at around 3,000 and you won't have any trouble.
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