pardon my ignorance but what are the advantages if a boomerang shackle?
No real "performance gain" with the Boomerangs. However, the shape does allow them to clear the rear crossmember on a YJ.. especially if the leaf springs are a little on the long side. They can also be used with a "hard stop" on the Jeep's frame, to help prevent "shackle inversion".. or kick back. Basically, the shackle swings inward during axle droop. Under the right conditions, the shackle can actually swing so far inward, that it "snaps" up against the frame.. locking the spring in place until the axle is drooped again (and usually pryed back down with a lever of some sort). The result of shackle inversion is often a bent main leaf spring.. in the area close to the bushing eye.
'80 CJ5 w/AMC360, T176, D44 w/Detroit 4.56, RE 4" YJ lift and 35s..