If you have a factory step bumper then you could use one of these:
http://www.amazon.com/Reese-Towpower-81 ... mper+hitch
The factory step bumpers are rated (I believe) 500/5000 although about 99% of the time you see real receiver hitches bolted to the frame. etrailer.com has a large selection.
If you're only going to mount an external box or maybe a bike rack this may work for you. Personally, I wouldn't want to tow anything with it as I would like a large margin of safety and would use an actual 2in receiver hitch.
There's always the risk that someone else down the road could assume that just because you have a 2in receiver (either mounted to the bumper or an F-Series hitch modified to bolt to the van's frame) that they can tow a heavy trailer. This can be disastrous.
If you want to go big (1500/10000), this one is great:
http://www.amazon.com/Draw-Tite-41945-C ... ords=41945
If you want something okay (500/5000) then this one:
http://www.amazon.com/Curt-Manufacturin ... ords=13053
There isn't much of a price difference (the cost of a few lattes maybe) so I would go big then never worry about it again.
If you're not in a hurry then hit up your local Pick-n-Pulls every couple of weeks. Look for the E250/E350 vans with the factory hitches. Those are rated 750/7500 (I think). If you find one then also cut out the factory 7-wire connector as far back through the wiring harness as you can go. Makes for wiring up a 7-wire connector later so much easier.
Hope this helps™,
Ray