I have a 1993 ford f150 with the straight six. Lately I have been noticing that under acceleration there is a vibration. If I push the clutch in and just roll it is smooth, or if I am up to my cruising speed it is smooth. I have checked the u joints in both front and rear axles and everything is tight. I know my front axle joints are worn but the front end is unlocked so nothin up there is moving. Anybody got any ideas?