What Monte said is the real engineering answer. WHENEVER torque is applied through gears, there is a component of the force that is trying to separate the gears. It is impossible to design a gear tooth that does not do that.

The more rigid the housing, the more torque the gearset can transmit. Most pinions are cantilevered, only supported on one end. When the gears start to push apart, the pinion is moving away but also bending away a little. The Ford 9" and other differentials that use a pinion support bearing on the inside are stiffer because it's harder for that bending to occur. Check out big truck differentials, many of them have the pinion support.

Unfortunately for the street, the pinion support takes up room that could otherwise have been used for a beefier limited slip unit. For the track, a spool is a better choice as it is shaped to not run into the pinion support.

There is no magic inside a race rear end. The billet caps can help as they are 2x stiffer and also stronger than the cast iron caps, but that is not going to prevent the pinion from moving away and the pinion shaft from bending away from the ring gear. As that happens, the tooth loading point moves farther away from the root of the tooth, and the tooth breaks. Just as simple as that. No magic involved.

The only solution is bigger/stiffer or a design more appropriate to the use, which in the Ford 9" is the pinion support.

R.