You can remove and replace all the injectors on the van with out too much trouble, except for the number 3 injector (passenger side second one back) this one you will have to remove the top portion first to get it to clear the firewall. The rest there is no problem. The site above is a good site for information and another good one is http://ford-trucks.com
. Just ask your questions in the diesel section that applies to the year you have.
My thoughts on it are possibly a fuel delivery problem. Fuel pump fuel filter? I would think if there was an injector problem you would notice it with a rough idle. The exhaust back pressure seems like a very good thing to check first since yo can do so without spending money.
You might be able to find a user on one of those forums with a scan tool or buy a autoenginuity to scan it yourself using a laptop.