Engine/Tranny Vibration

I am having some vibration issues with the transmission in my 99 windstar

When I approach 60km/h the vibration starts and at 70km/h the vibration gets worse but if I go faster to 80km/h or above the vibration goes away. Also, when I get to 70km/h with the vibration happening I hit OD OFF, the transmission shifts and the vibration goes away. If I drive around for a while the vibration will come back at about 70km/h with OD OFF. Also when I am driving with the OD ON and the vibration starts I hit OD OFF and nothing happens but most of the time when I do hit OD OFF the problem goes away. What am I looking at here, the torque convertor? or is the 4th gear the problem?

Thank you, DTF

I forgot to mention that the tranny shifts fine and the fluid was changed like 5000 km ago and the fluid is at the proper level.

Have you ever checked the powertrain elect harness for chafing against the ac tubing? .. its a long shot, but an important one.

Possibility of motor/tranny mounts?