You raise more questions with your symptoms---------how is the air flow affected from the front "vents"---what are you calling vents: floor, dash or defrost? Is the air flow cold, warm or hot, does the temperature control change the air flow temperature?

Re: rear air----does it default to A/C full time or simply not much heat?

Perhaps seems like a silly question but is the coolant level adequate to fill both heater cores?

 

JWA is correct on "flow" being a major clue. Your year van has vacuum controlled ducts and valves for the HVAC system. Adequate vacuum is a big problem on older models due to leaking connections and control parts in declining health. I've discovered a few areas that deserve a look from owning my 94. I've attached a page from the service manual with notes.

 

^^^^ That's a great drawing for the vacuum system---I've used mine a lot of times looking for a missing vacuum signal.