Basically, pre-millennialism is the view that the second coming of Christ precedes the 1000 year reign. Post-millennialism, on the other hand isthe belief ingradual improvement and enlightenment of humanity which would lead to the millennium at the end of which the second coming could occur.
So, JWs are quite definitely pre-millennialist - definitely not amillenialist.
I'm not sure that post-millennialism continues in anything like the same form in which it become popular during the nineteenth century. It was associated with a lot of speculation about the place of America in divine providence. Many of the Puritan settlers believed that the society they were creating was God's new Israel and this view gained momentum with the formation of the Republic. This is the era which gave us the Battle Hymn ofthe Republic - Mine eyes have seen the glory of the coming of the Lord.
Rob Crompton http://snigsfoot.blogspot.com