So long as they are not a progressive wound coil spring, it is fairly easy to calculate the rate. A qwik google turned up this on-line calculator:
Pro Shock Coil Spring Rate Calculator
Another that might be a real bonus when fine tuning time comes along:
Sway-A-Way Calculators
If you want to see the math going on behind the scene:
Suspension Coil Spring Rate Design Equations Formulas Calculator
Others:
F-O-A OFF-ROAD Shock Absorbers
Coil Spring Rate Calculations by Wallace Racing
I believe that the air shocks mentioned are an air spring built into a monotube shock. Quite robust, but typically those are not good at carrying much weight or going very fast (temperature largely varies the spring rate). They are quite popular in light weight crawlers as they are simple and effective for that application.
What a friend has talked of doing, but has yet to actually do, is to put a single (large?) air spring at the center of the rear diff. It would carry the weight w/o having a huge effect on articulation. FFT