Vibration when deceleration?

My Car was previously owned by my Grammy who rarely drove it in the last years of her life. It had been impeccably maintained when I received it 4 years ago but due to age, a lot of things had to be replaced. As of last month, it has new brakes, axel, suspension, and in an effort to figure out the vibration, a new transmission. Yet, the vibration continues.

It happens in a very specific range. Once the car accelerates to 30 mph or above, there is a vibration that feels like driving over a grate when decelerating. The vibration occurs when the car slows to 25 MPH and ends when the car reaches 15 MPH. The car brakes normally and there is no other sound or smell or anything associated with the vibration.

Three mechanics have tried to uncover the cause and the motor mounts are being replaced today. No idea if this will solve the problem.