Is there a difference between different years of PDM's, (stock units). and is it possible to have a bad pdm without having it stall/miss/hard starting issues? I read the article someone else posted about the hummer PMD and it mentioned poor fuel economy as an issue with the stock PDM. So I guess my question is can a PDM affect fuel economy but not give any drivability issues?

No

PMD normally does not throw a code. Usually you have drivability problems when they go bad. Never replaced one for poor mileage, but it is remotely possible.