Thanks for your responses guys. The grumble doesn't increase or decrease with speed. The noise and vibration seem to be coming from below my feet but I know that apparent noise location can be deceiving at times. The wheel bearings have been replaced/inspected recently and are in good shape. I cannot recreate the noise with the truck on jack stands with the tests you outlined or any other tests I've tried.
Sometimes when it happens in gear, I'll put it in neutral and coast to a stop. The grumble will stay constant in or out of gear and will continue until the truck comes to a complete stop. At a complete stops the grumble always goes away but it will grumble right up until a complete stop. The I'll put it into gear and start of again and it may or may not come back.
I drove it today for six miles and it only happened once from a stop and then disappeared after a few seconds.
I'm thinking it's something in the transmission but it's the intermittent nature of it that has me scratching my head.
James
