The car at the bottom of this post has had narrowed stock 31 spline axles in it for about 25 years now...I haven't raced it every year but I have close to 1000 passes on the car over it's stay with me and the axles look like new...
Given the weight of my car and the terminal velocity, I would estimate, in low gear, the engine is applying a total of 6200-6500 lbs ft to the rear assembly...which of course is split between the two axles....
Sound stockers are fine for most applications....once you get in the 10's at an NHRA track, you have to use aftermarket axles anyway...
Pat
http://www.jps.net/binay/webdocs/racecar_thm01.JPG