Thats right  here in Australia  you can buy a kit of 2 red type heavy duty
air bags that fit inside the existing springs, you then run a heavy duty
hollow plastic tubing to a air filling unit that can be increased at any
service station or from a home air compressor. The cost varies from car to
car  but the base kit is about 350.00

Hope i helped

Peter

Hello group:

 I have a 1991 Mitubishi Montero LS 4x4.  Recently I took the family
camping which involved loading the cargo compartments, filling a good
size roof carrier, and putting 3 bicycles on a carrier on the spare
tire.  When loaded, I found we were bottoming out on the stops on the
rear suspension while on rough coastal roads here in Oregon.  As I
hope to eventually add a tow package to this truck I was hoping
someone could offer advise on the addition of air adjustable shocks as
opposed to a heavy duty shock.  I am hoping to avoid the ultra stiff
ride with daily driving, but have the support that is needed while
loaded or while towing.  Thanks in advance