When a unit of vehicles moves, they all must move at roughly the same speed, e.g. they all move up to 6" or they all move up to 12" etc. Once all the vehicles have moved, they need to be within 2" of each other, just like ordinary troops. Only mobile vehicles stay together like this; if any of the vehicles in the unit are immobilised for any reason they can be ignored and left behind by the others in the unit. Treat any vehicle left behind as a separate unit for the rest of the game.
Moving
Source: 3rd-A-Palooza (3.5) Rulebook v1.4, p. 43