Gravity is not only created by heavenly bodies like stars and planets... Ok, don't even bring in that planet, Even then if the truck would change it's velocity, the box would move. It is beacuse of the gravitational field created by the moving truck. Changing velocity of the truck would change...
My friend, light is not instantaneous. Suppose your friend lives next door, around 10 metres away from yours.If you hold a 10 m long stick in your hand, and poke him, that surely will be instantaneous. The same happens with electrons in a conductor... Their speed is in picometres. Then how is it...