The title of The Grapes of Wrath is taken from an old song called the Battle Hymn of the Republic. There is a line in there that says:
Mine eyes have seen the glory of the coming of the Lord:
He is trampling out the vintage where the grapes of wrath are stored;
He hath loosed the fateful lightning of His terrible swift sword:
His truth is marching on.
This is a hymn of freedom from slavery, which both main characters suffer (of a sort).