Ok, looking at the 03 service manual, page 14-2, it appears that fitting is for a check valve on the gas-powered trucks. In gas applications, there is a hose that connects to that nipple and runs over to the "liquid expansion chamber". It's part of the evaporative emissions system, and there are actually 2 of them on the tank (one toward the front, as in the picture, and another toward the rear). It is UNUSED in the diesel application, and should be plugged. Perhaps DCTECH got one with the wrong nipple installed from the factory, and it wasn't plugged?