My 1992 Gr.Voyager. 3.3L has started to shake when accelerating when reaching approx. 40 50 mph. it subsides when reaching 60. Ideas?

Had same problem vibrated between 30-40 on acceration only. Had tires balanced 3 times no different. What fixed it was new CV shafts in front did not have poping noises like cv joints normally do just the 30-40 vibration, the shafts fixed it!