The freon lines for the Front/Rear A/C units are common.
I'd check the temp valve on the rear unit that opens/closes the heater line to the core. As for running at idle and the A/C warming up some this is common with R-134 systems in vehicles that have electric cooling fans for the engine. HOWEVER, the suburban hasa belt driven one. That said, the clutch on the fan could be slipping too much and not allowing the fan to run at a speed which pulls enough air thru the condensor. Also your charge could be weak. Check out the valve 1st (inside the right cover in back) making sure both switches in the roof are set to COLD 1st. Then set the unit to run on the FRONT SWITCH and turn it to HOT. If it switches over then turn it back to cold and set the selector to BACK CONTROL. Then do the above all over again. If the valve on the core doesn't cycle then look for either a vacuun leak or a bad valve actuator. Once you get that fixed THEN take and check or have the charge checked for full/low charge.
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