Ed,

For any automobile gears, you need a high-pressure lubricant. I don't know
how they got away with ATF for that job. For hypoid and spiral-bevel gears,
as in a rear-axle diff, you also need a lubricant with high shear strength.
That's what heavy rear-end oil is supposed to have.

If by shear strength, you mean viscosity, then ATF might be quite
viscous in low temperatures mentioned elsewhere in the thread. Or, are
you thinking of shear thinning or other non-Newtonian/viscoelastic
phenomena?

Bill