Did you catch it when their truck almost tipped over?
After the Joads had stopped at the Colorado River at the California/Arizona border to take a dip, once they got going again, did you notice that climbing back onto the road from the shoulder the truck's front driver's side wheel actually left the ground?!
You could hear some short vocal outbursts of concern which would be expected - but I couldn't tell if that was the actual recorded sound of the incident or if it was just recorded and looped in later.
Somehow, I can't imagine that they scripted in that popping-a-wheelie scene so it must have been rather exciting for all of those people on the truck when that happened. Another one of those "real-life" accidents that just happened to get captured on film.