While I totally agree with what Sniper said I have run Air Shocks for years on many of my cars.
2 tricks I use.
I run two separate lines to the shocks so air won't migrate from shock to shock.
Do not fill to the limit. They are OK to supplement the spring small amounts. Running them all the way up and jacking the rear end to the sky is asking for the shock to tear the mounts loose.

That being said, and because sniper is correct, what about spring re-arching?

When I was younger the cost of having the springs re-arched used to be pretty comparable to getting a great quality shock.