It would be a lot simpler to fit a set of air bags inside the rear coil springs, and air 'em up when a little more capacity is needed. Otherwise, buy a good kidney belt and be prepared for a rough ride all the time. The same thing could be accomplished with air shocks, but they have a bad habit of overstressing the upper and lower shock mounts, and they require much higher air pressure to carry the same amount of extra load.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!