I live in the DFW metroplex and before I start driving I let my 2011 ranger 2 wheel drive 4.0 v6 warm up all the way and the idle get below 1k
but as soon as I go from park to drive the temperature sensor drops to almost dead cold.
And even when driving it won’t completely warm up all the way.
Any thoughts as to why.

 

Believe you've got the later ThermoStat Housing with one ClipMount TempSensor\Sender\ThermoCouple
instead of early Housing styles that had two ClipMount\Threaded TempSensors\Senders\ThermoCouples.
Sounds like a bad TempSensor or TempWiring or TempGauge\Cluster; could be a bad\sticky ThermoStat.
Any recent ThermoStat or Housing or Coolant system maintenance or changes?
Radiator & OverFlowTank levels?
Coolant smell or leaks?
Coolant leaking\pooling behind ThermoStat Housing under IntakeManifold?
Do HeaterHoses get hot & have CabHeat when you have that switched on?

If new ThermoStat Housing needed, recommend Aluminum Housing, instead of FactoryOE\AftermMarket junk leaky plastic.
For more info click here.
Video of changing ThermoStat Housing on a 2011 Ranger here:

 

Thanks for the vid! My 2001 varies around also. It doesn't go all the way to cold but it moves around a lot. Planning on doing the stat and aluminum housing upgrade and the two funky hoses this Spring.

 

a coolant flush approx 3 weeks ago.
problem happening before flush hoping it would fix the issue
radiator and reservoir are full with no leaks or smells
No fluid leaks in the engine compartment

will be doing the sensor change this weekend
do i need to drain any coolant when changing the sensor or thermostat?

 

Yes drain maybe a gallon to get system level below top of thermostat housing, then you can open up the housing without spilling much coolant.