From Surly the maximum tire clearance on a 29er is 2.55". After reading a bunch of other forum posts from other sites asking the same question, I never read anyone going wider than 2.5" on a 29er. I don't think this will fit between your seat stays.
If you ran 700c you could get probably get wider clearance on the rear. When you really start pushing the limits of tire width, you'll notice any wobble in the wheel because it'll rub your frame! Here's an article on how to measure tire clearance. According to Surly, when you put the wider tire on the Karate Monkey, there's an issue of it rubbing the front derailleur. They make a spacer for that if that becomes an issue for you!