Thank you for asking your question on JustAnswer, the most common cause of the problem you are having is a defective Temperature blend door actuator. (CLICK HERE FOR INSTRUCTIONS). Hope this helps you out.
Before replacing the Blend Door Actuator, make sure cooling system is full and there are no air pockets trapped in the system, also while the engine is running make sure both heater hoses are very hot, if one is cooler than the other suspect a plugged or restricted heater core.
If the hoses are only getting warm, (must be very hot) suspect a defective thermostat, as long as there is no air in the cooling system.
Make sure that there are no leaks also, any leak will lead to poor heating performance as well.
A/C Electric Blend Door Actuator
The A/C electric blend door actuator is located on the plenum assembly.
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No you will not have to take the truck to a dealer, and if you need directions Just (CLICK HERE).
As far as price is concerned, the labor is .08 of an hour, depending on shop rate per hour. And the actuator is less than $100.00.