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I just put on 31x10.50s and the clearance at the bumper when I turn is less than an inch.
Common wisdom on these boards is that 32x11.50 will fit the F150 4x4 without mods - but I think there would be some rubbing as mentioned above.
If you're willing to cut or space out the bumper and cut the front fenders and put on bolt on fender flares, you ought to be able to run your 33s. You may need to change to a wheel with a slightly different offset.
I would not run anything bigger than a 32 11.50 with no lift. If you do decide to lift, don't go with a body lift. Unless this truck is a farm truck or a beater, don't cut it.