There is a noticeable shake in the front end?

1994 Plymouth Grand Voyager mileage: 64,000. Guys, I need some help. I have a noticeable shake in the front end that occurs at about 60 MPH - but only if accelerating. The faster the acceleration, the more noticeable the shake! Speeds above and below 60 MPH, no problems. I have replaced the front tires, struts and mounts, had new tires examined and rebalanced, and a front-end alignment. What should I try next? HELP! Thanks