Try this get it back up to the speed it shakes and shut down the engine if it's still there check the wheels for a bent rim the tires for out of balance the driveshaft could also need to be balanced. If it goes away with engine off check the mounts engine and trans. loose bolts to the bellhousing. And then there is always the option then in the rush to finish you never tightened down the shifter all the way. could be a bad bushing in the shifter if yours had them. Go with the easy stuff first and if you can't find it toss on some new muffler bearings
