The only legitimate way is springs with an increased arch. I too have used longer shackles but they have several bad effects. I was younger and dumber back then.

A set of custom springs isn't that expensive. You can pick the spring rate you desire.

Adding a leaf to your existing springs will increase the spring rate meaning the rear end will ride harsher. IMHO it's just another crutch, unless you regularly carry moonshine!

Whatever you end up doing, try to keep the front and rear pivot points at the same relative height from the ground. Change the relationship too much and you get into self-steering effects. For example, my '64Dog rear was set up, stock, so that when the vehicle tilts one way, the rear end steers in the same direction. It isn't much when unloaded, but I do a lot of mountain driving and having the rear start steering itself towards the outside of a curve is kind of spooky.

R.