REAR AIR SUSPENSION SUNK?

That system is a real %GER&&^V to work on. Basically you have 4 ride height sensors that talk to the chassis control module. That module then tells the ride control module and the compressor and it's control solenoids where to add or subtract air so that the car will ride level with the load in it.
From the description I would check the control fuses but expect to find that it's either a bad height sensor of the control solenoids that are part of the compressor. The rear height system is the last image. If the fuses are good, look for the control module and see if it's taken a bath. Then look at the compressor and it's controls, because if it's running it could still be bad or the control head is not under control of the height system. However there are powers and grounds to test in the schematic.