Seal leak 8 in rear end

I hope someone can suggest an addtive for my 1966 mustang third member to stop a leak by the yoke. Soon I will have to pull the axles and pull the pumpkin for a rebuild . I know they make additive to stop leaks for engines but 8in rear ends. Is the seal installed by removing the yoke or is it installed internally before the gears go in.

It is installed by removing the yoke, but once you do that you will need to remove the outer bearing and install a new crush sleeve.