03 lb7 fuel pressure, boost and power problems. HELP.
 

So I've got a 03 2500 Silverado. Recently I noticed that she had went from running just fine to having a huge lack of power. Examples being, while getting on the highway she smokes like a train and takes forever to get up to speed. Actual fuel pressure and desired are so far apart it isn't even funny, desired being 17-20 on scan gauge 2 and actual being 4-6. At idle desired and actual match. When coming down an off ramp desired is actually Lower than actual. Truck is totally stock with the exception of a straight piped exhaust from turbo back. Codes being thrown are over boost, low rail pressure at idle and enrichment. Maintenence is done regularly and religiously. I'm new to duramax problems as I also own a couple ford's and those seem to be my problem children with this duramax purchased new in 03 and never a single problem. Truck has 180,000 miles, most of which are highway miles no towing, she's a pavement princess with a hitch for looks. Always run a fuel additive. Shes garaged every night. I was thinking fprv or fpr but then someone said it could be cp3. I have a otc, scangauge2 and a few other cheaper devices. I'm curious to know what device I need to be able to command things to do what I want them to do, like command full rail pressure at idle and such. Being a mechanic isn't a problem and am very familiar with being one but again it's always on ford's or pre 07 tractor and trailers that we own. Any advice is appreciated.