There are two possible problems i believe:
-If idle drops low when stopped (500 rpms or lower) then most likely the cause of the shaking is the low idle. Find what is causing your car to idle low.
-If idle is normal: sounds like you have an automatic transmission hydraulic pressure or engagement problem that is causing tranny to not engage all the way. Could be valve body, pump, or clutch pack issues. Will have to be inspected by a tranny expert.
-If idle drops low when stopped (500 rpms or lower) then most likely the cause of the shaking is the low idle. Find what is causing your car to idle low.
-If idle is normal: sounds like you have an automatic transmission hydraulic pressure or engagement problem that is causing tranny to not engage all the way. Could be valve body, pump, or clutch pack issues. Will have to be inspected by a tranny expert.