My shackles did the same thing when I first tried installing my springs. My Rough Country kit instructions mention that this may happen (Yeah, yeah, I know, I know, Rough Country kits aren't expensive, but I have had no problems at all). What is happening here is that the arch of the new springs causes a shorter effective overall length spring eye to spring eye, which makes it impossible to pin the springs to the rear shackles without having them in the "rolled forward" position, and they will stay there if you lower the vehicle onto the ground that way. To get around this, use two jacks, one on each side, positioned just ahead of the spring pads (the rear shackles are bolted, but not tightened and the U-bolts are loose to the spring pads). Now, jack each side up a little at a time with the vehicle weight on the front arch of the spring, this takes some of the arch out of the springs and in effect "lengthens" them to a position where the shackles are now in a near vertical, or slightly rearward position. Now, install the U-bolts and tighten them down to the axle, which holds the springs and shackles in the correct position. Finally, lower the vehicle onto the ground and the shackles are "captured" in their correct position and they will stay that way. As the springs flatten out over time, the shackles will roll rearward (like the 20 year old springs that I replaced).