When you start the vehicle, or apply some load to the rear, do you hear the compressor activate to level the vehicle. Most cases involve an inoperative level ride due to either a faulty compressor, or circuit faults that create an inoperative compressor. If the compressor is faulty, look for a condition that may have contributed to it, like split shock bladders or lines. If an exhaust solenoid fault is detected, it is integral to the compressor. Aside from hearing the compressor operate, you can also investigate and check the shocks for some pressure in the bladders, it should have some.