What would you call the fear of big trucks?

There isn't a phobia specific to big trucks, although it could be covered under Megalophobia, which is a fear or large or oversized objects.

Most people who have a fear of large trucks don't actually have a psychological condition. If you're going down the interstate in a compact car (or even full sized car) and you have one of those riding up your tail end, it's not a clinical phobia - it's a legitimate reason to be concerned. And, quite frankly, it's a fear more people need to have on the road, because then maybe they wouldn't be so prone to cutting right in front of us, slamming on their brakes right in front of us, etc.