Krawlers, I have seen first hand their superiority on rocks (thats all I run, so I'll comment on that aspect only) over swampers. the swampers are a good tire, but don't flex well and are so heavy they sap a lot of power. But, from what I've heard, the Krawlers are heavy, too. Around here, the only sound hears from a swamper is the thud-thud-thud-thud like a card in bicycle spokes from them slipping on rock. I would choose MT/R's second. i know a few guys running them on their sammy's and they are a great tire with awesome sidewall flex, also. As for size, 37's on L/C axles are fine, so go as big as possible. I have a l/c FJ-40 rear behind my junk, and it is very stout, I was amazed.