LB7 issue. Undercarriage of truck is soaked with diesel fuel.


I require some suggestions for an issue I have just encountered today. I have a 2002 GMC Sierra crew cab with the Duramax LB7 engine. I did a trip today (120 miles) and the truck used a 1/2 a tank of fuel! When I fueled the truck up I noticed diesel fuel on my back bumper! I then looked under the truck and in shock I saw that underneath the truck it is completely soaked in diesel fuel??
The truck has a "rough" idle, sort of like a miss to it. The truck at idle always ran smooth. Do I have injector and or injector pump issues? Any suggestions will be greatly appreciated. This is the first time that I am posting on this site but have been a member for a number of years now. A great site by the way.

Cheers,
CareyK