Before I put new 32's on my Sammy, I had run quite a few times at Paragon on my stock birfields without detonating one.  Last time out we ran Rattler (Black 1 Rating) in the rain and put a LOT of stress on the drivetrain, and they held up fine.  Haven't had a chance to wheel it hard since I got the 32's, but from all I have read and seen, if you stay at 32's or under, the stock birfields will hold up. ÂÂ
Oh yeah, btw, a well built Zuk will run circles around most vehicles offroad. Biggest drawback is the extremely short wheelbase which can make for some interesting wheel stands on the steep stuff. I've found that my Zuk on 31's was able to outperform several other heavily modified vehicle on 35's when it came to crawling.