My boss Jason Demello has a dual case in his FJ. I believe it was one of the first ones to ever get it installed, but his is the auto
You dont need to relocate the tank and you are able to use that existing case. I believe a guy in Tennessee has a dual in his FJ with the 6 speed. Its going to cost you ALOT. Your going to need a new coupler along with an output shaft.
I would suggest swapping out the whole case with one you can switch to 2wd.