Cryo treating will not change the morphology of a Quench and Tempered axle shaft - so no it won't do squat for it.
Secondly, that failure is NOT a defect. I could whip out a copy of my spline shear stress calculation to show you that the stress concentration at the root of a spline tooth is extremely high (due to the high "k" factor that splines hve on stress calcs), and that ANY car could break those axles - maybe not the first time, but the cumulative fatiguing of the shaft at the spline root would eventually snap it like that. That break that Demon posted pictures of is a text-book example of a shaft fracture at a spline transition.
Like I said the only way to beat this phenomenon is as follows:
1) Larger spline diameter (not possible in this application)
2) Better spline root to relief transition (i.e. smoothing of the spline root) - expensive
3) Better quenching with a polymer instead of water to increase toughness and improve grain structure
4) Use of a 41xx series steel with higher concentration of chrome and molybdenum - also expensive for a production car with a common driveshaft across all V6 MTX lines.
5) Reduce shock loading
You need to remember (or learn if you're not an engineer) the effect of impulse/aka shock loading. i.e. Applying 100ftlb of torque to a shaft over 1 sec is 10x less damaging than applying it over 0.1sec (i.e. like in a launch at high rpm's)
