I've been asking the same question and have gotten a good answer.
You will have to cut some metal, probably not too much though. I have a 2011 pro4x w/ 33s on stock wheels and 2.5" SL and 2" BL. I have yet to put my spacers back on since I bought my new tires a week ago. I'm still not sure if I'm going to or not, mostly because winter is coming and I'm using that as an excuse to be lazy. If I put them back on I'll let you know. In the meantime, ask Vorian what he did, in short, you'll have to cut back some metal that a stock mudflap would cover. Ask Vorian for a pic or two and you'll see the dirt line in the pic.
Hope that helps