Could be a stuck heater water valve for the rear air. Big question would be is the rear air dehumidified? I'm not sure if you can access the refrigerant lines under the vehicle, they may pass into the body at some point. See if you can find metal lines under the right rear of the vehicle. With the engine running and a/c cycled on, one line should be near boiling and one should be near freezing. (Don't confuse it with the heater lines--they both will be very very hot). The input is hot and the output is cold (you'd think it's backwards). That will tell us if the refrigerant level is an issue. How cold do the front vents blow? If you run it on max cold for awhile does the shifter feel like an ice cube?
Try accessing the rear air system.
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Originally Posted by AllData
1. REMOVE REAR SEAT OUTER BELT FLOOR ANCHORS
2. REMOVE BACK DOOR SCUFF PLATE
3. REMOVE REAR DOOR SCUFF PLATE RH
4. REMOVE REAR WINDOW SIDE GARNISH
5. REMOVE REAR QUARTER TRIM PANEL RH
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There are two heater hoses connected to a valve. Have someone operate the rear air controls, cycling it between max hot and max cold and see if it is switching. I can't find detailed information on the rear air for this particular truck, so I'm giving kind of a generic approach.