I have a Vibration that starts at about 50Km (30mph) and up. The vibration stops when I use the clutch and when costing down hill. There is no vibration when I rev the engine. I don't feel the vibration in my steering wheel, and I just had my wheels balanced. I feel it in the gear stick as much as the truck in general.

Check your universal joints and the tailshaft bushing in the transmission. Wiggle the drive shaft by hand and feel for excessive play.

i agree check your unaversles do you hear a clunk when you take off? and may want to check pinion bearing. if it's not the shocks skiping it would show your tires are cupping or chopping..

I'd be looking at the rear axle, generally symptoms of differential failure are similar to u-joints, where it is only under load, or sometimes no-load with gears. If your u-joints are good, you may be looking at some pinion bearings. Good luck with it.