As I said, some monsters are termed as bosses by the game, and are not bound by this limit. Since monster levels in Nightmare and Hell are exactly the same as area levels (with the exception of bosses), that means that only monsters that appear in an area that is lvl 84 or higher will be able to drop Zod runes. Treasureclasses of the same group 'trade places' with each other to match the monster level as closely as possible. 'Act 5 (H) Good' is level 85, while the next in line, 'Act 4 (H) Good', is lvl 82.
This treasureclass in turn is only used by the 'Act 5 (H) Good', which belongs to a large group of treasureclasses. Zod runes only drop from the treasureclass 'Runes 17'. I've pieced this together from what I've read regarding how the game work, but I do not know if I've missed a vital detail that disproves all of this. However, of course I want to figure out where these runes can actually drop from, and being that I don't want to be wasting my time with killing monsters that can only drop up to Cham (:P), I dug into the game files. With the awesome new drop rates of runes in the new patch, I decided I wanted to start playing some D2 again.
