It would have to be big rig hacking or physical cracking, not hijacking.
You can't exactly hold a gun to the driver's head and kick him out of the driver's seat if it's AI- (or even remote-)driven.
It would probably be very easy to get the trucks to stop, simply by creating an obstacle. Even then, for a hijacking, you need to gain access to the cabin (if there even is one), override (or prevent) the 'exception: vehicle stopped due to obstacle'-alarm from being sent, prevent all communication of position of the truck, make sure you and your material are unrecognizable for the shitload of live feed cameras on the thing and then perform some control override to get it to actually start moving towards wherever you're planning on stashing it.
Hell, for a successful hijack, it'd be far easier to just hack into the network of the transporter or do some 'social engineering' on an insider and direct the truck to wherever. Obviously not a job for ex-truckers.
Of course, the far easier alternative is just to get the trucks to stop in a fairly quiet place (traffic wise), break them open and transfer the contents to another truck. It would still have to happen fast and visually anonymously, of course.