Actually, I don't think it is. I see a handful of companies having options to cryo treat gears they sell for $70 to $100. An article I found from MotoIQ in 2018 says "Cryogenic treatment generally can improve the fatigue strength of steel gears by over 100%. Cryo treatment is relatively inexpensive, usually the gears and shafts of a transmission cost around $100 to process." I imagine rear end gears would be in the same ballpark.