The MaxGear 75W90 will work fine in your differential - it just does not meet the factory recommended viscosity grade.
MaxGear 75W90 is used in racing and high performance applications and has plenty of load carrying ability and protection for your Mustang.

Oils vary in fluid thickness by temperature. They thicken at low temperatures and get thinner as temperature increases.
This is measured by a term called centistokes (cSt).