I saw your video on the additive and I wanted to know if I can use it in my rear differential when I change the fluid, and if so how much if the Seal Fixx should I add .
Do you have a leak? Any "stop leak" product is going to be a temporary band-aid if it works at all. Scotty usually recommends ATP AT-205 "Reseal" if you want to try that sort of thing, but if it's not leaking I would not use any such additive.
No
even if it is leaking, then the cover gasket or the bearing seal should be changed. There is no reason to use such a product in an axle.