It may not be the best thing, but if you took the driveshaft out, put the truck in gear, you could have someone simulate "driving" the truck, and you would be able to lay directly under the tranny to hear if anything was wrong. If nothing happens, its either going to be part of a load problem, or you can eliminate everything and change the oil in your diff. Check your brakes (if that hasn't been said) because those also are directly linked to the speed of the vehicle.