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Mate that your body mount, a few of us including myself have had troubles hitting on this especially whilst offroading. Theres a few things you can do, if you can find what offset your running that'd help us figure out what would be best.
Thanks to the guys on here (Max and quinny) they found that a "0" offset rim works best with 285 tires and factory upper control arms, so your easist solution may be to try some new rims.
If your not willing to swap your rims, chopping the body mount is the only other option although you may still have to do this with 0 offset rims. IF you want to run 285/75x16 tire its a risk you have to take if you plan on doing hard wheeling. have a look at this thread Max put together with a few examples of whats involved:
viewtopic.php?f=41&t=7273
Hope this helps, Max