I had the same problem. I have resolved my problem by replacing clean air filter.
I think this is what’s happening even though I am not a mechanic,
The car computer control air/gas mixture as well as shifting automatic transmission gear. You may notice gear shift around 1400~1500 RPM in about 35 to 45 MPH. This is because the computer has pre loaded pattern controls the gear shift. The problem is that the engine at this stage “supposes” to produce certain amount of power to support either constant speed or shift the gear to accommodate the pattern. In my case, because the air filter was really dirty therefore affect the air/gas mixture and subsequently reduce the engine power output. This leads to two things
1. Under power for the speed/gear pattern therefore causing the vibration (as if driving a standard shift in too high of a gear).
2. Or force the engine to run in higher RPM for more power. This will lead to a different shifting pattern. After accommodated gear shifted the engine RPM come down.
Hope this will help. Please let me know it does or doesn’t help.