I know it's 11 years old and might need some attention, but injectors vs maintenance items is a huge $$$ difference. Also given the odd situation under the hood makes me think there is something going on, who would put a intake on a truck and not mess with more of it. I really don't want something thats been messed with by a diesel ricer or something. No stacks on it at least.
The more I look into it the more I think there is a problem with this thing, should it feel fast? I punch it getting on the highway and there is a 2-3 second lag while you hear the turbo spin up and then it goes a bit better than my 5.3 suburban. Not much better though.