I know exactly what it is, it's the lock in converter on your 4speed Auto. When you reach 4th gear on your Auto, the converter "locks in" so that the car puts the power directly from the motor to the tail shaft instead of through the torque converter.
Have a similar issue on my car around the 70km/h-80km/h range where there is a slight vibration, but this is probably due to my speedometer being out.
You will mainly notice the lock in converter just as it's starting to engage around that speed, and it will go away if you speed up or slow down between the 70km/h-80km/h range.
If the vibrations are very noticable it may be because your speedo's out. Get a mate to drive in front of you on the highway at 100km/h in another car and follow him in the EA and see if it shows the right speed. If the speedo is out significantly you will need to get the speedo gear changed on the auto(easy to do)
Other then that, you could just do what I do and ignore it... because it won't hurt anything.