Not sure why everyone is against the double shackle, going on 4 years running it and never had a single problem, I love it. I would do it all over again on my next rig in a heartbeat.
You say you are running a 1.5" lift shackle, so how long is the shackle itself? I have found that a stock chevy shackle on the frame connected to a 6" "standard" shackle, set up to pirate specs (11" forward of the stock spring hanger when replacing standard 48" long rear springs) works excellently. No inverting, no axle wrap, and by using a small IFS bumpstop, no "frame slam" between the shackle and the frame.
99% of the time, people who experience problems with double shackles did not set their shackle angle correctly, or did not mount their springs and shackle hangers properly. Like I said, in almost 4 years running 63s with a dual shackle, I have never inverted them or had any problems. In fact, it is one of the oldest mods on my 4runner that I have not gone back and redone. It just works too well to warrant messing with. Anyway, do not get discouraged, follow the Pirate FAQ and you will be as happy as I am.
Here are some pics from when I first did my 63s several years ago:
Maximum possible droop, without axles, tires, shocks, etc attached:
Mocked up before welding per P4x4 specs:
Done, on 4 wheels, shocks attached and full standard weight load:
