Air bags and cat scale shock

Porpoising is often caused by a difference in suspension frequency, front to rear. Classically, you want the rear suspension frequency to be higher than the front so that it can 'catch up' after a bump. If the rear is the same as, or slower than the front it will contribute to porpoising. With a heavy load in the back, and a trailer beyond, you may have reduced the rear frequency too much. The only way to change it is to reduce the weight or increase the spring rate. All of the various suspension band-aids increase the spring rate. The only one that is easily tunable is air bags.

Shocks can dampen the oscillations and reduce the problem, but don't really address the cause.

There is a pretty good explanation of it here if you are interested in the theory.