Front unit is orifice controlled; the rear usually has an expansion valve so that's why you see the differences when turning on the rear. Expansion valve is thermostatically controlled so it's more ''sluggish'' responding controlling flow vs the fixed orifice.
Don't forget the rear is an air recirc. system only so it's not the main cooling like the front unit. It's more of a supplemental cooling.
Expansion valve in the rear will divert some of the ''freon'' around the evap. when no air is going through the evap. so front will be colder when rear fan is off.
Like oilcan said a full drain and proper refill is only way to get right.

 

Ok just to give everyone an update who has gave their inputs. So after changing out the low pressure switch and running the truck with the gauges coming up from the under the hood I was able to see that the pressures where getting pretty high and turning off the compressor, indicating that there was to much refrigerate in the system. So after a number of times of removing some refrigerate I was able to get the pressures in a good range, at 90* 40/210. After all of this I have drove the truck a number of times. The system performs the best it has ever been. The system never shuts off now under any conditions that used to cause it to before. I can run and shut the truck off, use the rear air and the truck gets down to 38* from the front vent. I was considering evacuating and changing out the Orifice and dryers and refilling by weight, but after how it is performing and the fact its been at least 20 years since I installed this compressor, I'm not going any further, pretty happy where it is at. So the way I see things, the LP switch was an issue along with over filled refrigerant causing the compressor to randomly turn off. Thanks again for the guys that shared their thoughts on this issue, it really helped me get these problems.

 

For future reference.

 

Brandon: thanks for reporting back the ultimate solution. 👍

 

I am glad we could help you, Brandon. That's what we are here for.