2000 Dodge Durango Heater problem

I have made sure the coolant level is full, however the truck only blows the warmest on the fans lowest setting. The higher the setting the cooler the air becomes. It doesn't matter when're the air is blowing either the situation is still the same. Dash or vents. The air is coming through the vents fine, just the temperature of the air is the problem. I know it is leaking coolant up into the inside of the hood so I already have to keep a close eye on the level but in the cold NY area heat is needed. Thanks for any help you can give.