I believe that Rangers kept the hot water valve way of controlling heat long after many vehicles have switched to blend doors. If there is a valve I would be looking under the hood under the hood. If it has a blend door, that could be the problem. It might be stuck. The blend door controls flow of air across the heater core and they can be difficult to get to under the dash.
The heater hoses should be HOT not warm, if the engine is warmed up. Take the heater hoses off carefully and flush the heater core with a garden hose. I recommend cutting the hoses off. If you stress the heater core pipes too much you will have to replace the heater core!