Hey Cas, no problems re the description, I think I got the idea...
OK, so you want to have a convoy that when attacked will split into two parts, the trucks that move on to the objective and the defending vehicles that stay behind and engage the enemy... The easiest way to do this is with a trigger...
Create a trigger that has the following in it's 'condition' field.
behaviour veh1 == "combat" or behaviour veh2 == "combat" or behaviour veh3 == "combat" etc...
This will look for the behaviour status of the vehicles in the convoy, the moment they go into combat mode the trigger will activate. Obviously you will have to have the convoy start with a behaviour mode of something other than combat, safe or aware should be fine...
OK, now you have the condition sorted... You now need to sort out what happens to the vehicles when they get attacked. To do this you need to split the group up. I'm going to assume the armour makes up the smallest part of your convoy... Say two vehicles at the front... (I recommend you put the armour at the front, see above post)
To split the group, put the following in the on activation field;
[veh1,veh2] join grpnull;
Then to make the two vehicles join each other to form the rear-guard force add the following;
[veh2] join veh1;
OK, you now have two groups, but what about the trucks..? Well, you can now get them to move away in what's left of the group they make. To do this add the following;
"_x domove (getpos *objective*) foreach units group veh3;
*("objective" can be anything you can name so a man/vehicle/gamelogic etc... DON'T use quotes)*
The trucks should move off and leave the armour to defend itself... Hope this helps, I've not tested it, but I have used something very similar. You may have to tweak the AI/rank settings for the vehicles, but you should be OK...
As for the armour at the rear of the column you can try it, but like I say I've had problems with it moving to the front of the convoy as it moves. The AI likes it there, something to do with having the strongest units at the front...