It really depends on how you drive. Are you a careful slow going driver or do you figure that having speed and power will let you make it up the obstacle just as fine as driving slow. You can really break anything with even stock tires on, it's just how hard do you drive stuff?
I have only broken two drum birfs, one with 33's one with 35s, both locked up front. Since I have done my fj60 axles with longs on the outside, I havn't broken anything, we'll see what happens when I get 38s. I'm just planning on upgrading everything as it dies, but i'm very easy on the skinny so, we'll see.
35s were fine for a while til the stock rear Dshaft twisted and snapped. Never had any other breaks though. Nothing too rough besides deep mud and the occaisional hill climb where I was living at the time. Utah terrain is a bit different than where I was. Just be prepared to start beefing it up as you break it, no prob.
I Broke one Birf with 35's,, Now i know how far i can push the truck till it will break and have since not jacked too much up.. Except i Twisted up my Rear driveshaft comming out of Cliffhanger in Moab, But i think it was due to a crappy Driveshaft shop using thin crap..
Now that i will be locking the Front, and running 37's i will be getting some Cromo Longs, Fine Splined Rear Pinon..