Originally Posted by KhanTyranitar

If you still have your owners manual, look at the scheduled maintenance, then bring it all up to speed. If the truck still runs like crap, then you can move on with diagnostics, but chances are pretty good that some TLC will fix the issue.

I would throw a couple ideas in, except you need to do the above first so this can be narrowed down, otherwise we all can produce a very long and potentially confusing list.