Unless you can feel a discernible engine vibration, this is probably just a "sympathetic vibration". The natural frequency of the shift lever is apparantly about 40 cps and it isn't well damped. Even a small engine vibration will add energy to the shift lever until it's going like crazy.
There are a couple ways to get rid of it:
You can stiffen or loosen the lever or its mounting system until its natural freqency is out of range of the driver.
You can add a damper to the lever to absorb the vibration energy. Maybe a heavy rubber sleeve that can be hidden by the shift lever boot.
But I ain't guaranteeing anything...