Good questions.
Sheared teeth could be the result of a number of things, but the basic premise is that the original vehicle was designed for a smaller tire with less traction, and other driveline parts that were smaller/weaker to insure that the breakage happened in parts that are easier to replace.
You can't do much with the factory parts.... the metallurgy is what it is. You can cyro the gears to gain maybe 10-15% more strength, but there's not much you can do if the pinion is deflecting under load (does the Toyota 3rd have a bearing support on both ends of the pinion?)
Ultimately, it just seems like a really expensive R&D project to try to find marginal strength improvements in the OEM design. I suspect this is why the really hardcore guys decide it's actually less work to do a SAS swap with a beefy aftermarket axle.
-G