I never noticed any severe cracking in the rubber boots on the ones on my '97 SLS, but I don't think I ever looked that close either. All rubber naturally degrades over time, and eventually, those cracks will become severe enough so as to leak and not hold pressure. I would add 50 psi of air to the shocks and see if they hold pressure.
The rear shocks (and front struts) are still original on the '97 that I used to own, and that my brother now owns. They ride fine and hold air perfectly. They were never subjected to much road salt. But I don't think it's "typical" for them to last so long in such good condition.
Now that you have them, I'd slap 'em in there and see what happens. If they don't work all that great, I'd seriously give up on used and look to the aftermarket. Monroe (to my knowledge) makes fully electronic air shocks, and Arnott and Boston Suspension make passive (non-electronic) air shocks. The air leveling still works, but they are no longer CV-RSS units. But there is a small chip in the shocks that causes the computer to still think the CV-RSS electronics are still installed, so they're plug-n-play.
The rear shocks (and front struts) are still original on the '97 that I used to own, and that my brother now owns. They ride fine and hold air perfectly. They were never subjected to much road salt. But I don't think it's "typical" for them to last so long in such good condition.
Now that you have them, I'd slap 'em in there and see what happens. If they don't work all that great, I'd seriously give up on used and look to the aftermarket. Monroe (to my knowledge) makes fully electronic air shocks, and Arnott and Boston Suspension make passive (non-electronic) air shocks. The air leveling still works, but they are no longer CV-RSS units. But there is a small chip in the shocks that causes the computer to still think the CV-RSS electronics are still installed, so they're plug-n-play.