The rear air also runs off the freon from the front air so more than likely you either have a restriction in the rear lines or maybe a temp blen door problem at the rear unit remove interior trim panel and see if the actuator that controls hot to cold door is working may be just a blend door failer.
this truck should hold roughly 5 lbs of freon.
-i once had this problem in a savana(same setup) and the filter before the dryer(front) was blocked up by someone that previsouly used AC line sealant. The ac lines must be opened to change this filter, unfortunately