kwrangln has a good post on shackles
Also,to get 2" of lift with shackles they would have to be 4" long.
4" longer, meaning 8" total. I ran 2" lift shackles and 4" lift springs on my YJ when I was SUA. It rode a little rough, but it flexed good, even with a 3" body lift, I rubbed 33s. However like already mentioned, those shackles get hung up on everything. I constantly hit them off-road.
As for the boomerang, already mentioned they won't let you invert a spring, they won't hit the rear cross member, and in my expierences they ride better (don't know why, just my opinion).