Has the EGR Valve ever been serviced? It may be sticking. How old are the fuel filters? How is the truck normally driven?
I know these may sound like odd questions, but I have noticed with mine, that when the fuel filters get near the 10k mile mark, I see a difference in performance and I start to get a light film of soot on the rear quarterpanel. I change fuel filters at 10k. Had the truck been driven "in town and slowly (grocery getter,socker mom)" driving style? When my wife drives mine in town to get the kids from school and run arrends, for a while, and I then drive it to work (30 mile commute half interstate) I notice the performance is down somwhat, and when I get on the interstate it will leave a haze the first few times. Then it will clear up and not do it till it is "the mom-mobile" again. I have also noticed recently with mine, I believe the EGR needs cleaned, If it idles for a few minutes, (like running into a gas station for a coke) the first couple of times, the turbo lags and will blow some black smoke throught first gear till the turbo spools up. I think the valve is strating to carbon up and is not operating at 100%. After the first couple of restarts it will clear up and not do it till it idles again.