check the ducting that leads to the back of the car for any leaks or clogs on the duct. if it is fine, then check blower motor on the engine compartment to see if it is strong enought to push air to the back of the vehicle.
My experience with 'air' tells me that the rate of release for the two valves would 'almost certainly' NOT, be the same. Some, or maybe all of the Chrysler expansion valves are adjustable... and can be adjusted to produce the proper balance of release. A balance between the two valves is needed to insure equal cooling front to rear..