I got a '98 Navi, same truck as yours and I was having the exact same prob with mine. I tried to get the codes read and they wanted to charge me $100 to do it. Of course within a matter of days my compressor burned out and it went downhill from there. I decided to do away with the bags and put some coils in the back, after alot of research I found http://www.suspension-alternatives.com/
I got 2 rear springs for $190 delivered to my door in about 3 days. The install was easy and only took about 2 hours doing it by myself. For now I left the air shocks in the front till I can scrounge up some more money, in the mean time I cranked my torsions up maybe an inch or two to level the truck out and I have no problems at all with it and suprising enough the ride quality is still awesome, if air ride is a 10, those coils are a 9. Hope this helps!