Can you lead-foot it away ahead of them? Or tell them that you're not going to drive in the line because you have a stop to make--and then pull off and kill 20 minutes? (of course, they might just stop on the shoulder and wait for you...)
Or just plain bring it up: Describe the situation that you described to us.
Tell them that this frightens you, that they would drive on the shoulder, and that they would make such sudden moves.
Tell them that it scares you if they aren't paying close attention to the road in front of them. And that you fear they aren't leaving enough space between cars for people to react to sudden situations.
Make sure everyone has really good directions and all the info they need. Including the cell phones of people in the other cars (not the drivers', or in addition to the drivers')
Then point out that everyone has this, people are familiar with the route, and that you personally would like the freedom to drive on your own without them watching you, and without the pressure to stay in a completely uninterrupted line.
(If they *want* to stay in an uninterrupted line, the way to do that is to drive below the speed limit. Then nobody will stay between you very long. But again, you don't pull off, you just drive at the steady slightly-too-slow speed until the other people get fed up and pull out--or until they turn off, if that's what they were doing.)
You can also put a big X on the back of each car using blue painter's tape, and then you all can spot one another even if cars do get between you now and then. Being able to see each other that way might ease their anxieties.