Have a pool of transports (trucks/halftracks etc.) that can be added/removed/re-assigned to/from units in deployment phase and left behind if not needed. Transports should be purchased and restocked separately from the units that use them.
Units should have a trait which kind of transport they can use (as with land vs. heavypull in PC1).and you should need to have sufficient transport units of a type to stuff a unit, i.e. you have to have 13 (unit s of) trucks to assign them to a 13 strength unit.
Thus meaning if you would have 30 (units of) trucks you can outfit two 15 strength units or 3 10 strength units with them.