Strange Vibrations in a 2001 Honda Civic

[anthro78]

So I’m driving my 2001 Honda Civic and it begins to vibrate like I’m running over a rumble strip. It lasts 1-2 sec. and occurs when I’m going 35+ mph. My mechanic thinks it might be the Overdrive in the transmission slipping when it tries to exit the gear. I’m hoping someone here might have an idea that’s less expensive than a rebuild…