I just wanted to finish up this thread and thank everyone for their input. I ended up just getting a basic set of springs and perches and swapping out the bags and leaving it at that.

Someday I will invest in the little black box that eliminates the air suspension inactive warning, but not today. I love this thing, but it's 16 years old with nearly a quarter million miles, and I'm not going to fix this issue, only to have the next weakest link break, and then the next, etc etc. When I get the time I will remove all the air components and sell them for what I can get.

Awntchi: I will keep an eye out for a diagnostic tool along the lines of what you suggest, although my budget is tight. Also, about the cause being a leaky airbag: while that is certainly possible (they look original), I doubt it, because after the system shut off it took literally 5 months for the bags to leak down. When I swapped in the springs one of them STILL had some air in it. If that slow of a leak is enough to burn out the compressor, then...damn.

Anybody want to buy a slightly used rear SLS setup? nonfunctional and only 230k miles! PM me!