Agree with checking the heater hoses.Note that there is usually a valve in one of the hoses that opens when the controls call for heat.
Also do a search on the forums for Blend Door Actuator. These have been known to fail and there are solutions with pics available.
As for the normal / low temp readings , this could be due to either a failing temp sensor , low coolant (an air bubble in the top end not letting coolant flow past the sensor properly) or a stuck fan clutch.
Check for low coolant by parking the truck facing uphill until cool and make sure the radiator (not reservoir) is full. You may have to check this a couple times to be sure.
Archimedes screwed us !
I knew a blonde dyslexic agnostic insomniac - she'd lay awake at night, wondering if there really was a Dog.
2003 Ranger Edge 3.0 manual 2WD