The early Jeep Grand Cherokee's have a problem with their air conditioning.
The door flaps that direct the air to either the window defroster, or the dash vents, are vacuum actuated. If the vacuum line or check valves go bad the air conditioning system won't work properly. A good sign of a problem is that when you accelerate with the air conditioning on, the flaps will improperly divert the air to the window defrost vents. When you let off the accelerator pedal and decelerate, the vent flaps will move and direct the air to the dash vents again.
I checked with my friend Gary, the senior mechanic at Don A Vee Jeep in Placentia, California. He told me to buy a new vacuum check valve. The valve drops in the vacuum line that exits the passenger side of the intake manifold (5.2L V-8).
I spliced the new valve in and it corrected the problem. If you are having this issue check all your vacuum lines for leaks. If you can't find any, buy this check valve and drop it in. The part number is #1-04677204, and it should run you about $24.00.
If you search the internet, there are numerous fixed for this problem. You will find a vent system flap door fix for this problem on eBay, I might try the check valve first, its a tad more economical.
The check valve is the small cylindrical canister, just below the exterior vent.