I will almost guarantee you that your problem lies in in the rear accumalator. You will have to split the air box in the rear of the truck that has the heater core and all the blend doors and there will be a small a/c accumalator. there will also be a brass expansion valve(same thing as the front orfice tube) with it. You can either go to Napa and buy a new one for $30 bucks or take the old one off and there will be a little screen on the high side of it that you can remove and clean. Because it will be plugged. Now just to warn you, the expansion valve deal is the easy part, splitting the air box is where the work is. You can also do all this without removing the rear heater core. I went through and 2 week ordeal with mine to figure this out but it makes ice in the rear now.