Mileage included in the rate only comes with one-way rentals. No company currently offers one-way cargo vans. I personally think it would be an excellent idea, though. Perhaps someone else in the future will start a rental company that does this. Currently the closest thing to a one-way cargo van would be a small box truck.

Perhaps try a one-way box truck from Boston to NJ and then take a train, bus, or rental car one-way back to Boston? Hertz' Weekender might come in handy. It's supposed to remove the drop charge on one-way rentals with a Saturday night keep required.