I won't presume to say that my suggestion is "great and powerful," but I will give it a stab.
What does the title mean? For one thing, it is from a verse in the book of Revelation: "The angel swung his sickle on the earth, gathered its grapes and threw them into the great winepress of God's wrath" (NIV).
It is also used in the first stanza of "The Battle Hymn of Republic":
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.
The Revelation text is God's judgment on a wicked world. The "Battle Hymn" speaks to the same thing, but in response to the savagery of the Civil War.
Given this information, how is the title The Grapes of Wrath appropriate for the story of the Joad family?