It is not posible to randomize the distance, the distance is defined per vehicle in the technics.ini, so all vehicles of that kind will keep the same distance.
But you can make convoys look a lot more realistic, by editing each vehicle independently:
Look for the convoy you want to change in the chief.ini and see what vehicles make up the convoy. Then lookup these vehicles in the technics.ini.
You will find an entry "best space" at the vehicle entry.
This is the space the vehicle will keep between itself and the vehicle in front. I changed this to value 32.0 as it seems more natural to me (in real time vehicles don't tend to drive to close together, especially when containing ammo and an air threat exists).
If you mix up the vehicle types in the columns (chief.ini) you can create gaps in your columns (eg give the jeep a best space of 50 and place one between a load of trucks).