In "The Grapes of Wrath", what happens to solidify the family as they drive along?This is when they first start driving to California
The family has already seen what happens to people like Muley Graves who refuse to deal with reality. Muley has lost his entire family and, by self admission, turned into "a graveyard ghost", as his last name implies.
Then at the beginning of their journey, the family must deal with two blows that force them to unify as a group. First, the death of granpa, who had stubbornly refused to leave the farm, unites the Joads in their grief and in the realization that to survive, they must continue their journey. Secondly, the Joads are treated as outcasts by the rest of society. Consequently, they are forced to unite in order to fight against people who want to close their eyes to the social injustices the Joads are facing.