@peterk9 "Neat things"........
Took me a LOOONG time to find this out, and it was by accident when scrounging around once under a Ford F-series 4X4 I noticed the front differential pinion gear was ON TOP of the pumpkin center section. IOW, the pumpkin was upside down!
I already knew about gear teeth and such, and that ring gear teeth are much stronger in one direction than the other.
Looking at the lowest few teeth above, the tooth profile difference is easily seen: the right hand face rises nearly vertically from the bottom of the tooth, while the left hand face rises upwards at an obvious angle which is sloped. The vertical faces are termed "DRIVE" faces, the others "COAST" faces. This means the drive faces transmit the forces to move the vehicle in the forward direction, "seeing" the highest forces, while the coast forces transmit only the force needed to keep the driveline turning when the throttle is closed.
Thus, drive faces in the REAR AXLE are really driving faces. But, the 4X4 design has the entire axle, in the front, turned around, causing the front gearset to be reversed also, and the forces to move the vehicle FORWARD are applied to the usually COAST faces.
So what? .....Gear teeth always try to "spread" themselves apart. The vertical DRIVE faces produce almost no spreading. Forces applied to the COAST faces force the gears apart. Such was a fact of life in early 4X4s, on up through the 1960s, and perhaps '70s. At that point, after many front axle failures, manufacturers began the unusual process of installing the front axles, UPSIDE DOWN, to make the old COAST faces again do "coasting duty". This placed the pinion and it's bearings higher up, clean out of the "pool" of lubricant they sat in in the REAR axles. Why they took the final step.......I am not sure. Because it confused many who did not understand all this tooth bullshit, or the final "fix" used:
They began cutting ring and pinion gearsets as "reverse rotation", to be used in plain old rear pumpkins for use "up front"!