You would more then likely have to pull them all to change 1 injector, because you have to pull the fuel rail to access them. I really think there could be another problem. Have you had it scanned for codes, if you can get the codes, post them so we can help direct you.
Here's the code: P0431
We just purchased this van last week. :(
Before purchasing we had it diagnosed and checked by a trusted mechanic. Today it was checked by the same mechanic and he mentioned that the catalytic converter is not functioning properly. It sounds like its the forward one on the #2 cylinder. Right at the exhaust manifold.
I'm sure you have check engine light on. Do as the Professor has said. Try to get a real scan with a real full enhanced scanner. With a real scanner one will be able to look at the serial data. This info is just as important as the trouble codes.