I had the same/similar problem with my last Explorer, a '98. In my case the
light came on after about 30 minutes constant running, loaded or unloaded.
Never, in the 3 years that I had the car, did I ever hear the compressor
running. I spent loads of money at my Ford dealer trying to fix it. I also
bought full set of self-levelling suspension 'hardware' from a breakers yard
and swapped it all over to no avail. (Shocks, compressor, ride height
sensor, valves and other gubbins). The problem only disappeared when I
traded in the '98 against my '00!
That reminds me - I'll stick all that stuff on eBay sometime soon!

Simon H