Hello, diagnosis by guesswork really doesn't solve anything. You can get lucky, maybe 5% of the time. Make an educated guess. But what in the transmission do you think is vibrating that fluid will fix?
If you are accelerating or decelerating and the vibration goes away, indeed, one of the things that could be happening is engagement or disengagement of the torque converter at the speeds you suggest. But, the problem is - you don't know that for sure, and if it is accurate, the question is why?
Fluid changes should be done regularly in accordance with manufacturer requirements anyway -- "Normal" driving says inspect fluid only; "Severe" driving says change fluid every 30,000 miles.
I would change my fluid, if it was my vehicle, every 30,000 miles. It is true that fluid that has broken down may not have proper friction modifiers for the torque converter and other components to operate at their best. But this is a low percentage guess, honestly. I guess for normal maintenance if it's been longer than 30,000 miles since the fluid has been changed, it's a good idea anyway. I would just say that it's unlikely to get much of a positive result.
By far, the best way to replace the transmission fluid in this vehicle is by using a transmission fluid changing machine.
Seek guidance for diagnostic testing using proper diagnostic equipment that will be able to monitor the operation of various components to see if there is any evidence related to the vibration.
