I had that same vibration and for some reason my clutch seemed to be difficult in reverse (48k miles at the time). It drove me nuts and my research suggested the motor mount. I purchased Pedro's remanufactured mount and I am very happy with the results. Pedro's top notch and he makes it as easy as possible to change this out.
Did it myself in about 3 hours - very satisfying when I was done.
www.pedrosgarage.com