Well first I tried Macguba's suggestion. It worked. And then without trying it on the 'filtered' mission, I tried a few of Acid's suggestions like Guard. Although it didn't work, you've shown me the wonders of Guard, and I thank you for that. Much more effective, isn't it?
And I've sorted out the problem now. And I think you'll shoot me when I say it, but I'll say it anyway. The waypoint after 'Get In' Truck led the Urals directly up that giant hill next to Saint Pierre. I now know that you cannot treat this bit of land as if it were flat when mission-editing. If faced with a waypoint to go directly up a steep hill, a vehicle will not move AT ALL. It will turn it's wheels once, and then nothing will happen. So, I just made several waypoints along the way.
Coincedentally though I've made my mission hard with all those Eastern groups, and I don't know if it works yet. ;D
I'm heading to bed now (12am here), and I'll check tomorrow.