I'm on 35" tires, and stock axles. I have not broken them... yet. My 4-door tracker is about as heavy as it gets, and it sees Rubicon use. But, I'm pretty gentle 'cause I don't like breakage. I'm going to carry a $30 birf spare from someone like trail-gear or something. If I start breaking often, I'll worry about upgrading to something more stout.
Look into axle trussing - especially near the ends - there's mini-truss kits that protect that weak point. Lots of info out on this (trail-gear, low-range offroad, bobby long, marlin krawker...)