Does the grumble increase/decrease with speed? My first thought is a bad wheel bearing. Can you tell if the noise is in the front or rear of the truck? I'd suggest putting it on jack stands and rotate the wheels individually and see if it's a bearing. Then put it in gear and rotate the rear wheels and see if it's coming from the chunk or transmission. If that does't tell you anything, start the truck, put it in gear and let it turn the wheels and see if you can isolate the noise that way. I recommend a 2nd person to help and be a safety observer during this step.
