As far as I can tell, the Immunity 31 and Sturdiness 31 enhancements don't exist in gear. Unless they turn out to be a schematic drop somehwere, I don't think we'll actually be able to get our hands on these enhancements. I think we'll be stuck using the Bastion and Bulwark enhancements instead.
They don't drop anywhere. There is no such thing as 72 crafting. PT and juggs want high mitigation mods while sin tanks want high endurance b mods. It is insanely easy to gear up a sin tank as it is.