This is a very common problem with some of the late model Ford overdrive transmissions. The source of the problem lies in the torque converter clutch that chatters as it engages causing a vibration to be felt.
While this would seem to be a major warning sign of failing transmission, this will usually be cured by servicing the transmission (changing the fluid). At this facility, we will also add an additive called Lubegard (black label) which has helped.
If the symptoms persist, follow the recommendations of you local transmission facility. The most important thing is not to delay having the service performed. The longer the unit exhibits the symptom, the more on a chance that damage will occur.
