I have a 2008 with 6.4l with about 80,000 miles. A few days ago, it suddenly lost power while driving on highway. I had it towed home. It continues to start and idle roughly, but will not rev up. The only code is a P0670, which is a glow plug malfunction code. I changed the fuel filters, but there was no change in problems. All stock, no mods.
Following up on my original post, the problem was the EGR valve. After I changed it (easier said than done), the engine started and runs fine. The EGR valve was stuck open. Hope this helps someone else.