I'm not sure how it would have got clogged, or even if it is clogged. Checking with a thermal cam is one of the methods you would use to rule this possibility out, to avoid the parts-cannon approach to problem solving. Casting sand is a common cause of clogged heater cores and radiators. Also, really old coolant that has turned acidic, or reacted chemically when someone topped off with non-compatible coolant. There are tons of possibilities.
I did a coolant flush early this year on my truck and the wifes durango. I did both coolant flushes in the same weekend. The next weekend, the water pumps went out on BOTH vehicles!

I DON'T think that was just a coincidence

. It doesn't take a mathematician to calculate the probability of it being coincidence, but 2 coolant flushes resulted in needing 2 new water pumps, at the same time.
It was Jan 5th. I did a quick search because I even posted about it, here:
I'm not sure as to what specific temp Mopar programmed them to open/close at. My truck doesn't have those.
Yea. same here. I like to test things before replacing parts, to avoid that as much as possible.
At less-than highway speeds, your engine is under less load and since there is less heat, a properly functioning cooling system can remove heat more easily, with just the fan. At higher speeds, there is alot more load on the engine so more heat, but also more airflow through the radiator, so, it depends on what is wrong with your cooling system.
Is your truck an electric only fan, or does it have a clutch fan? My 2013 has both, and i don't think i've ever heard the electric fan come on. I'm sure it's probably happenned, but i've never heard it come on, or noticed that it's on. Not once.
I know the feeling.