The shackle mounts on a Silverado or an F-150 are inverted to what they are on a Ranger. By adding length to the shackle, it raises the leaf pack higher above the shacke mount. When the length of the shackle is decreased, the truck gains lift because it pulls the leafs down closer to the ground. You would just have to measure them but i really don't think you'll get much drop as your stock shackles are already pretty short and leave little room to remove any length. They are a direct bolt in requiring nothing but a shackle though.

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2005 FX4
TIGHTENED T-BARS
LONGER REAR SHACKLES
265/75/16 FIERCE ATTITUDE M/T