CST doesn't recommend a 15" wheel, but if you go with a low enough backspace, it will push the wheel far enough out that it will clear the spindle. I have some pics on here some where of my truck with stock z85 wheels and tires on it with the lift spindles, they just cleared the spindle by like 1/2" , but the bulge of the tire past the lip of the wheel had it maybe about the same or even less from the upper balljoint.
Again to re-emphasize what others have said the stock z85 wheels are only 6.5" wide and that is far to narrow of a wheel to put any larger tire on. BFG used ot make a 33x9.50x15, but they no longer offer it, it might have been ok on the stock wheels, but I guess we'll never know now.