Thermostats can affect average engine temps by overcooling the engine when heat production is less than cooling capacity, giving you more thermal headroom under full power, assuming you're not just driving flat-out 100% of the time.
Imagine you have a car with a 180F thermostat and not enough radiator driving around a course with uphill and downhill sections. It easily cools to 180F on the downhill section, but by the top of the uphill section it's creeping higher and higher every lap to some scary temperature we don't like, say 230F.
Now if we put a 160F thermostat in it, it might cool to 160F by the bottom of the downhill section, and stabilize at 210F by the top of the uphill section. It seems to solve a heat problem, but it's at the cost of accelerating engine wear. It would be better to find and fix the actual problem.
Not sure if it's standard on the 5.9, but zj's have a heavy duty towing mechanical fan clutch available. I put one in my xj.
As I type I realize you have dual electric. I'll leave there for others.
I think if you can put an air dam, extending down from that splash shield, it should help to create a low pressure area to pull hot air from the engine compartment.
Shouldn't affect arrival angle and may help mpg's.
To restrict coolant flow to my heater core, I installed a 14mm cheap 1/4" drive socket in the heater hose next to the nipple on the intake, and clamped it in place with a hose clamp. I used one that didn't have the high-quality square-all-the-way-through broached drive hole, but had a smaller, round piece of flash in there. Since I installed it, I haven't failed a heater core during burnouts (I had failed 3 before that), and I haven't split open the heater hose at the intake at high rpm (I had that happen twice).
In reply to NoBrakesRacing :
5.9s are single electric fan only. Basically the much-swapped Taurus fan with a slightly different blade and shroud.
