It could be a couple things, but I'll list them in most likely order:
1 - VGT Vanes Are Stuck/Otherwise not functioning. This can happen when your truck sits for any extended period of time, idles a lot, or you just baby it, and don't exercise the turbo. You need to run the crap out of the turbo every once in a while to keep it clean and functioning properly.
Solution: Pull and clean the turbo, check all connections, and the vane solenoid. Not too hard, just takes some time, search the forums for how to do this, there's guides with pictures and everything.
2 - FICM/Injector Problems - See the Injector link in my signature for information on this
3 - High Pressure Oil System problem. Check your ICP and IPR pressure.
My bet is on #1 though.