2003 Ford 500 Xlt 4 0 V6 4x4 Model Vehicle Shutters 35mph 50mph Sometimes Very

I have a xlt 4.0 v6 4x4 model. My vehicle shutters at 35mph and or 50mph. sometimes its a very light or medium shutter. My vehicle is current on its tune up, trans fluid change and i keep up with oil changes. there are no check engine light or o/d light on that would indicate i problem. . When I take it to the trans shop they cant feel the shutter because for that date and time it shutters lightly but i feel it because i know my vehicle. I would like to get to the bottom of it before it gets worse.Do you have any suggestions