Sorry I agree with lostrider, the air inlet or recirc doors do like to break on this model. [obviously a poor design] Actually I have ran into another door that breaks, [either the floor or panel door] this is probably what is going on with your weak air flow problem. It is actually located in the dash and I can not find any pictures of it..
If you have under 75K on your truck, definately contact ford for repair assistance, they will help you out.
Ford Motor CoCustomer Relationship CenterPO Box 6248Dearborn, MI 481211-800-392-3673 [FORD]www.customeraskford.com
Open glove box and look under dash to see if any of the vacuum motors are moving and the end they are connected to is broke off.