When you say dragging.... Do you mean it feels like something in the back of the truck is slowing you down? Also have you considered spinning the front wheel and listening for a dragging noise?
I recently had to replace my front guide pins for my break shoes. Turns out the grease in there was all dried out and the shoes/breaks would stick causing my breaks to stay engaged while driving. This was my dragging issue.
Now, Slowhand also has a point. Jack your back end up and spin your wheels to see if they are harder than normal to spin. A note for this, you will hear a slight metal on metal sound when you spin the tires IF the breaks are still holding a little. Keep in mind it probably won't be a high pitched whistle but a metal sliding noise. You will know it as the wheels shouldn't do it at all when they spin.
