This is a very common complaint with the 06's, especially the Megacabs. You might want to make sure your mix door is functioning properly. They have a tendency to break at the pivot (there is no stop for the door). I had to have mine replaced when it finally fell all the way off and blocked air flow. Haven't used the A/C yet this year, so I hope it will make a difference.
Overall, it seems to me the HVAC on my Mega is very subpar compared to a Chevy Extended cab we have at work. You could toast marshmallows in the winter and make ice in the summer. Just seems to work better.