If the shocks are too short and you unload the rear suspension rapidly you'll overextend them, which could blow the shocks apart. Or rip the mounts off. Or both. Catastrophic failure is probably unlikely, but with them being over 2" shorter you could definitely ruin the shocks.
The other thing is, why do you want air shocks? If the purpose is lifting the car, you'll ruin the shock mounts. The upper shock mounts were not intended to support the weight of the car, just the resistance that comes from the shocks. If you raise the car with the shocks you're putting the weight of the car on the shocks, and the shock mounts will fail eventually. There's a lot of cars out there with screwed up shock mounts because of the whole 70's air shock craze.
If you want to raise the rear of the car, look at getting the rear springs re-arched. Or new springs entirely with a higher arch. Air shocks and extended shackles are band aids, and both will cause damage in the long run to raise the car in the short term.
