this is technically ver,very,very hard to do.
I had a basic idea, although this is probably not possible. When the at gun were 'attached' to the truck, the AT gun is deleted or destroyed, then a new mesh is spawned, one of the Truck with the AT gun attached, although this way would negate actually having a 'hinge' and would be purely for looking like it is being towed, the gun is detached and a new at gun + new truck is spawned, this would be done without explosions, so the truck/gun would disappear for a second then reappear. Pretty bad way, I know but seems to me the one with the least work involved.