The truck drives fine until it gets up to about 40. Then once it hits 40 in 3rd gear, it will start vibrating and shaking. It almost feels as if it's coming from the transmission? Or possibly the motor? I don't know, and haven't been able to pin-point the problem. But once I get out of 3rd, and shift into 4th and get up to about 50, the vibration goes away. And that's the only time that it does the vibration and shaking. Any ideas/answers? I could really use the help.