electronic load leveler

I have a 1999 Gmc Jimmy with 160,000 miles. It has the envoy package(dont know if that means anything). I noticed that the rearend has been sitting low and i know that it has air shocks. I never hear the compressor kick on to level out and also the accessory compressor for filling up tires and what not also does not work. Today I checked the fuse and its fine I also found the compressor inside the passenger quater panel. Im lost as to where to go from here and any help would be great.

Thanks again
Chris