Woah woah woah.
Trying to get the car to not move when in drive and the brake off is not only dodgy but illegal* and most likely it would make your problem worse. (hills aside)
The rack damper is just to smooth out the idle speed. If you idle is up and down all over the show then by all means sort that out...otherwise don't touch it.
Just dial up the idle until the shaking stops. RPM not so important as long as it's not overly high (say 900 RPM+). You'll know if it's too high. Bear in mind that dialing the external idle up can mess up other things and you might find the revs not dropping as fast as they used to when letting go of the juice pedal...
Not really a slush box person but that's my take on it
(*think in certain states its illegal but I have no idea...)