4 runner vibration

I have a 1998 4-runner with a vibration problem at freeway speeds, 4 cylinder engine 215,000 miles. The problem has gotten considerably worse in the past day, I had my transmission drained and refilled to no avail, when I am on the freeway if I take my foot off of the gas while in gear or take the truck out of gear the truck will start to vibrate, if I put it back in gear and accelerate it goes away, I would really appreciate some advice or pointers so I can ask a mechanic what to possibly look for without sounding like I don't have a clue, possible driveline, cv joint problem?
Thanks a lot! Dave :(