Weird AC Problem

[2010 Eddie Bauer]

Good morning all; I thought I would try to get some insight on a weird AC problem I am having. For background, I have a 2010 Explorer with the 4.0 V6. My AC system was to my knowledge untouched (I am the third owner of the car) until two years ago when I had to have my evaporator core replaced. The system was recharged at that time after the new evaporator core was installed. I have the single-zone manual climate control. I replaced the blend door actuator myself about one year ago when it failed in the cold position. Here is my problem: when I turn the front AC system, but not the rear AC vents, I get Luke warm air from the two driver's side vents and cool but not ice cold air from the passenger side vents. It will get marginally better over time, but not near as cold as it should be. If I turn on the rear AC vents on in conjunction with the front and set both controls to the same fan speed, it will cool down to the upper 50s temperature wise according to a vent thermometer with an outside air temperature of 90 degrees Fahrenheit, with the driver's side vents being slightly warmer than the passenger side vents. If I shut off the rear vents, the front vents start blowing warmer air. Is this an issue of improper vaccume pressure, vaccume pressure being lost at certain times, or a weak AC compressor? Thank you for your help; I am taking some classes to become better versed in this sort of thing, but still have a lot to learn.