Its probably in the computer, so you'd have to get chips custom programmed to have the rev limiter removed, or at least that would be the right way but expensive.
I'm guessing the tach works either by voltage or resistance, I don't know which or if its either one, you might be able to rig up a circuit of your own that would cut off the fuel pump. I don't know how well that would work though, it might make the van stall out which might be worse. Don't ask me how to do either one since I don't remember enough from electronics, but radio shack sells cheap easy electronics books that should tell you enough to get it done.
But anyway, if it works by voltage I would use a pot(poteniameter, i'm sure thats not how its spelled) to set the right resistance in the circuit to cut off at the rpm and speed you want it too, rather than playing around with a bunch of resistors. If you do try that and get a book, or already know about electronics you know what I mean.
Superchips makes custom chips, their site is www.superchips.com I think. That would probably be the best but expensive way like I said, but you could probably get a discount since I guess you'd need a lot. Just tell them what you want it to do, and that you dont want them to try and improve the performance. There might be some places around you that can reprogram ford chips, like the dealer, but I don't know how all that works.
I just got home from a trip and the rental car my boss gave me had a limiter at 3700rpm. It was a gran Prix with a 3.1L V6. You could hold it to the floor in neutral and 3700 was all she wrote. It was limited by spark control. You might want to talk to a rental place because most limit their vehicles.