Non-mopar tech question, sorry...

You might be right about the heater core under idle antifreeze pressure is low allowing coolant to go through a semi-clogged core. Under more rpm drive antifreeze pressure up it takes the path of least resistance and bypasses the core through the built in bypass in the core. It is kind of common on Rangers. I had the same problem with our shop truck. There is a product sold at GM dealers that will clean a cooling system it is largely made of phosphoric acid. It does an awesome job of cleaning but it will also be deadly to weak gaskets.