Ian
The shocks are an easy DIY fit just like any other car (with the addition of
a simple push fit airline fitting on each shocker). Only probs might be
siezed bolts in the bottom bushes. As you can hear the compressor running
sometimes when you have stopped (actually, it should be just after you have
closed a door) the system must 'think' it is working and adjusting the level
correctly. The compresssor NEVER ran on my old '98. It might be worth
getting under the truck and lubing the ball joints on the suspension level
sensor arm and checking that the multiplug is making a good connection. it's
located on the rear axle just to the UK driver's side of the diff, a sort of
lever/arm arrangement going vertically up from the axle to the body. The
ball joints are similar to those on throttle linkages or tailgate gas
struts.
As for the sitting low at the rear thing. My '98 Explorer had self levelling
suspension plumbed into all 4 corners. My 2000 North Face only has self
levelling at the rear. The thing I first noticed when I collected it was
that the 2000 North Face stands higher at the front than the '98 ever did.
Maybe Ford tweaked the front suspension to compensate for saving a couple of
quid installing self levelling at the front? The front standing taller may
accentuate the saggy back end...
BTW, my local Ford agent diagnosed 'leaking seals' on the rear shocks and
quoted me over £400 fitted. I acquired the parts I mentioned before for a
fraction of that and it was qute clear that the shocker seals were perfectly
good when I disconnected the airlines. I also bench tested the shocks and
they held pressure for several days.
I hope you have better luck getting it sorted than I did...

Simon H