That temp is low and while lower outside temp can effect internal engine temp somewhat the thermostat should be maintaining a higher temp.
Thermostat in your engine should be 192degF
This means it won't start to open and send coolant to radiator until coolant in the engine reaches 190degF, thermostat usually starts to open within 2degF of rated temp, at 192degF this t-stat would be fully opened at 207degF, about 15degF above rated temp.
Newer engines, 1980 and up, get best MPG and best oil cleaning when run with coolant at 200degF.
The outside temp can matter because engine metal is hard to keep warm and the 4.0l is known to be a cold blooded engine, it is hard to keep temp up in very cold climates.
Your thermostat could be open a little even when engine is cold, could just be old, thermostat should be changed with coolant just like rad cap, but that is owners choice and usually is fine not to change it.
You can test if it is allowing coolant to flow even when cold.
Start engine cold, pop hood and feel upper rad hose, it will be cold.
Drive for a couple of minutes, 2 or 3 minutes, and then stop and feel upper rad hose again.
It should not be warming up with the engine, thermostat is closed and no coolant should be flowing in the upper hose, so it should still be stone cold.
If it is warming up then thermostat is open a bit.
Engine Cooling system is a closed system, radiator is there only to get rid of excess heat, in warmer weather it will be used all the time, but in colder weather it may never be used, may never even get warm, it is just for excess heat.
Good read here on changing thermostat: How To Replace the Upper and Lower Thermostat Housing on a Ford 4.0L V6 SOHC Engine
It has lots of pictures.
One other thing that it could be is that the ECT(engine coolant temp) sensor is starting to fail, so reporting the wrong temp to the computer, if you can get a hand held temp sensor you could check that against the ECT sensor temp you see on your reader.
In the above link the housing has an ECT sensor and ECT sender, yours will just have the one ECT sensor, Ford combined them.