In one sense, the entire novel consists of journeys, and so writing about journey themes looks easier than it actually is. The main difficulty you’ll have in your essay is deciding which aspects of the journeys in the novel you’ll wish to cover.
One possibility would be focussing on the notion of physical vs. spiritual journey. You could focus on Jim Casy and see his journey, and eventual death, as parallel to the travels and sacrifice of Jesus Christ, with whom he shares initials.
Another possibility would be to focus on journey as loss. Just as much as a journey moves to a destination, it leaves something behind. For Granma and Grampa, whose connection with their land runs deep, the journey represents a loss of self; their displacement results in their death. You could address this theme in terms of bio-regionalism and eco-criticism.