I agree with Phatocaster - could be the blend door actuator or the recirculation door actuator. I have a 2005 SLT and it seemed as the the AC was not cooling off very well AND it was always on high fan speed. I took it in to my mechanic, he pulled the system down and reinstalled the freon and dye. Found no leaks, just 1/2 lb low on freon - not bad after 11 years - it blew cool air. He went further and found the answer to my question about not feeling, or hearing, any difference when I push the recirc button. Found power to the recirc actuator but the door was not moving - it was stuck in the fresh air position, hence why the air was not cooling off after driving for hours! They wanted $400 labor and $180 for the OEM part! I found the OEM part on Ebay - if you are going to buy this part BE SURE it is the OEM part with the CEI stamped on the case - for $50! It took me about 6 - 7 hours but the door now works like new!
One thing I found was that the recirc door actually is variable on it's opening/closing, meaning, it's not just fully open or fully closed! Makes a big difference now! ALSO, some people will confuse you with the term blend door! There are actually 4 of these actuators, if you have the dual zone climate control. There IS a blend door actuator, which is located between the heater core and the evaporator, and there is the recirculation door actuator located above the glove box and WAY back in towards the firewall! The other two actuators are towards the floor, one on the driver's side and one on the passenger side.