Originally Posted by
Nelaphim-EX
Shipyard is a different set of missions and a different 'hidden requirement' - I believe it has been narrowed to requiring level 3 Shipyard with a certain number of the ship equipment unlocked.
To get the 6.2 Garrison Rare missions you need to be at or above 675 Item Level and have numerous followers at 675 Item Level (Somewhere between 7 and 9 minimum.)
Mind you, this is all just what people have put together, Blizzard is not so kind as to give open answers on the requirements.