Many people don't really understand what a shackle reversal actually does. We all know it moves the shackle to the rear, but many don't understand why. The basic theory behind it is that it allows the axle to move backwards over bumps, making the whole motion smoother.
On a spring with some arch, the leaf spring will "grow" as it compresses, like so:
Under normal circumstances this would force the axle forward into the obstacle. This image shows how the axle travels with the shackle in the front. The axle is forced forward into the obstacle making the whole motion rough.
However when the shackle is relocated the spring "grows" towards the frame moving the axle rearward. This image would be like your typical shackle reversal system:
Since the axle travels backward it helps reduces some of the shock, as the whole motion is more natural.
However this all changes when running springs that are basically flat.
If the leaf spring is flat, it starts out at its longest point, therefore it will always shrink as it compresses or droops. In this instance the axle will travel rearward when it goes over an obstacle.
Ok so it would seem this is a no-brainer modification, right? Not exactly. Shackle reversal changes the way the Jeep behaves quite a bit. With shackle reversal, when braking the Jeep will have a tendency to nose dive harder than a Jeep with forward shackle setup. When you apply the brakes the axle will be under a rearward force, forcing the springs to compress, amplifing the nose dive. Shackle reversal may also cause interference issues with the body. As the springs compress, the tire be forced backward towards the body, instead of away from it like in a forward shackle configuration.
A new front driveshaft is usally required too with shackle reversal. The issue is getting a driveshaft with enough travel. On a stock setup, as the front compresses, the driveshaft compresses, but at the same time you have the shackle moving the axle forward. Same thing again on extention, as the axle droops, extending the driveshaft but under normal circumstances the axle is moving rearward when drooping. However with a shackel reversal driveshaft travel is amplified. As the suspension compresses, the driveshaft is forced to compress, but the axle is also moving rearward now so the driveshaft is forcd to compress even more.
A shackle reversal setup does make it difficult to bend springs. With a shackle forward setup if the shackle hits something it can be forced rearward, bending the spring, like so:
Shackel reversal does eliminate this problem.
As with any modification there are pros and cons. Best to read up and consider everything before jumping into a shackle reversal. This link will further explain advantages and disadvantages,