I recently bought a 2006 CRV auto trans with 155k miles on it. It looks like it was well taken care of for the duration of previous owner. I noticed that during acceleration only between 15-25 miles per hour that I get a shaking/shimmy/shuddering under the driver seat. There is no vibration in the Steering wheel at all. It starts at about 15 mph, peaks at its strongest at about 20 mph and tapers off until it hits 25 and then nothing. Trans shifts smoothly.
Things I have done in the past few months after buying the car:

2 drain and fills of trans fluid with Honda fluid about 1k miles apart.
drained and fill of rear axle. honda fluid
exchanged some power steering fluid. honda fluid
new pads and rotors on front
changed serpentine belt
changed drive side mount hoping that would help but no difference.

The previous owner had the front axles and shocks replaced along with new tires shortly before selling.
I have driven this car for about 5k miles now and it drives great other than that particular speed. Highway speeds there are no vibrations at all. So, Im a bit perplexed. Just trying to see if its anything I can or diagnose myself before bringing it in.
thanks

 

Check the rear transmission mount for rips. Check all your motor mounts actually... Also check to see if the exhaust is touching the frame