Ticking noise under truck

[87r10]

Hey folks, I have this problem and it's driving me nuts. I get a ticking sound from under the truck that is only noticeable when I'm next to a barrier or another car, don't hear it sitting still with the eng. reving or not, so don't think it's an exhaust leak. It only happens when in gear and moving. Truck is running fine. Seems I've heard it may be U-joints or speedo cable. Have any of you run into this? Thanks