What happens to solidify the family as they drive along? Of what significance is Grampa Joad’s death? How does Granma take it? What is Ma’s philosophy of “holdin’ on”? What is the value of Casy’s prayer?

You need to shorten your questions or submit each one seperatly. There is too much information to get to on one post. I'll do your first one here. 

Despite his tough veneer of anger and bitterness, Grampa dies from a stroke. Since he was the one family member most adamantly opposed to leaving their home, it was likely the separation that hastened his demise. Casy makes a direct correlation between Grampa's death and their journey, reinforcing the idea that these people have a significant personal relationship with they farmed.