hey guys. got a question i can't figure out. my truck has a very distinct vibration that feels like it's coming from the mid section of the drive line, about half way through the truck. but, it only occurs at 1250-1350 RPM. whenever the engine hits that range, it vibrates, whether accelerating, decelerating, or coasting, if the engine falls into that range, it vibrates. and i can't figure it out. Help.....Please!!!!!!