The shackle is what attaches the leaf springs to the frame in the rear of the truck. One common way of raising the rear is to put longer shackles on thus pushing the frame up higher. To figure out how much lift you will recieve from after market shackles, measure the shackles from bolt to bolt on the stock and after market shackles. Now for every inch longer that the after market shackle is over the stock shackle, you will recieve 1/2" of lift. For example, i ordered Daystar shackles which were 3" longer than stock from bolt to bolt; therefore, I recieved 1.5" of lift in the rear of the truck.
Alway research and see if shackles are right for you. There are other ways of getting lift in the rear, such as add-a-leaf. Both accomplish the lift but in different ways. The shackles can affect the ride of the truck. I have not noticed any difference but others say the rear will tend to be loose or sway more than it did. For those that that will take their crew cabs off -road, there is one benifit that the shackles will give your truck. It increases articulation.
There are different brands and types of shackles. The ones shown here are non-greaseable Daystar shackles.
Instillation was simple. First you jack the suspension up right at the rear in front of the leaf spring. Make sure you keep the truck level on both sides. ALWAYS USE JACK STANDS! When there is enough arch in the leaf springs, you unbolt the stock shackles and then just bolt the new ones right back on! Just make sure that you jack the truck up high enough to get the leaf springs to line back up with the new shackles.. I used wooden blocks to get it up higher than my jack could go. The whole process took about an hour.
Here is the stock shackle up against the new shackle.
The new shackle is so long you can easily see it sittiing below the body of the truck now. But now the rear sits 1.5" higher than it did before.