there are a LOT of variables in play here. terrain, driving style, tread pattern, locked/open, truss, etc. A dana 30 is way tougher than its name implies, but it has limits. personally I wouldn't go bigger than some 33" mud terrains, but I'm a cautious person since I usually need to drive the truck home. I put a 44 in the front of my truck because I wanted more of a project than just a bolt-in 4wd conversion.
and for every guy that runs giant tires on a dana 30 and doesn't have any problems, there's a guy like me.. who cracked a ford 9" case in half (we think by dropping it on a rock), so nothing is invincible.