You can do a quick an easy test after you check the coolent level by feeling the hoses after its warmed up (be careful as the hoses my be very hot). Let it run just sitting in driveway or garage for about 15-20 mins. (you want it at normal operating temp) Then run your hand along the hoses feeling for any temp changes between the upper and lower Rad hoses. Then the same along both heater hoses. If any feel considerably cooler than the other then there maybe something wrong with the thermostat and or the hot water valve going to the heater core.
If they all feel about the same (hot) I would start looking at the blend door. If I remember correctly they use a stepper motor to control it and there were some problems with them.
2001 Ranger Edge 3.0 V6, 5 speed M/T
Limited Slip Differential, James Duff Traction Bars
Kumho 30x9.50 15 MT / Black 297 Series 15x7 Rims