While I was able to find a few rental companies that allow one way rentals on moving-type vehicles - links open in new windows - (
of them would allow a one way rental with a cargo van.
Unfortunately, this is common. I've been contacted by many people about this very topic.
I've never been able to find availability for a one way cargo van rental. I believe that with those types of smaller moving truck rentals, like cargo vans, the companies must prefer to keep their cargo vans local.
Probably because they're rarely used for more than a day or two, and are probably in constant demand... unlike the larger type trucks which tend to be used for big moves (which mostly take place on weekends, and/or at the very beginning or end of the month).
With Budget Truck Rental and UHaul, the smallest one way vehicle available is a 10ft moving truck; with Penske Truck Rental the smallest thing available for a one way rental is a 12ft moving truck.
Hope this helps a bit!