If it is an injector you may be able to tell which one is not working properly if you can get ahold of an infared temp gauge. You can point it at all six exhaust ports when the truck is running and the one that is most different is most likely your culprit. I did this with an injector issue on my old 99 ford 7.3, but it wasn't blowin black at idle just running real rough and under-powered. It can't hurt to try anyways.