Not to harp on you or anything, but this question, well, all of your questions that you just asked, have been asked and answered at least a couple hundred times...maybe more. The search function is your friend, but, to answer your question:
285/70/17's may or may not rub. They shouldn't rub at all on the street, and again, they may or may not rub offroad, it just depends on the tire and your FJ in general. My KM2's, which are 285/70/17, on aftermarket wheels, do not rub at all, I have not chopped my body mount.
This brings us to the subject of the body mount. The body mount that the tire would rub on, if it did at all, is located just under the fender flare closest to the door of the FJ. It is attached to the frame, and does what its title says, mounts the body to the frame. For some reason, Toyota decided to have the FJ body mount stick out quite far into the wheel well, but it can be trimmed and capped with no adverse effects.
Hope that helps...