I'm getting a weird vibration. It happens for only about 1 second at a time but can happen many times during a 10 mile drive. It sounds the same at all speeds. Doesn't happen when under hard acceleration, or when off the throttle completely. It only happens when barely into the throttle, like when trying to keep a constant speed. The vibration feels like a subtle version of driving over a rumble strip. Already checked the u joints. Truck has 123K on it.