Shackles are a vital part of a leaf sprung suspension. Flexing of the elliptically-shaped leaves changes the straight line distance between the spring pack's mounting eyes. A shackle is placed at one end of the spring pack to allow for this change in length.
Wheel travel and ride height can be increased, within reason, by increasing the length of your suspension's shackles. This is a relatively simple and cost effective way to fine tune your ride's ride. Furthermore, a shackle lift will not increase axle wrap like the other budget lift option, a block.
Pinion angle and the swinging arc of your suspension should be kept in mind if a shackle lift is considered. Changing the length of the shackles will affect the angle of the pinion on that axle. This can be good or bad depending on the application. Also, the path that the wheel follows when the suspension cycles, the swing arc will be changed. Research and careful analysis of your setup should help you determine if a shackle lift is right for you.
We recently found a shackle to increase the ride height of the rear of our heavy duty dodge diesel made by Truckin' Suspension Lifts. The shackle option was a good choice for our application since it would not affect the spring rate or increase axle wrap. The long wheelbase and small relative lift of 1.25 inches meant that we would not be drastically changing our pinion angle or suspension geometry.