These “sensitivity” settings don’t really change sensitivity per se, they just change how the measurement is reported so you can choose between a wide measurement range or more decimal points.
I think the only reason you’d want to change this would be to increase the measurement range to prevent saturation of the accelerometer. I suspect that the accelerometer is already set at the least sensitive setting to give the largest measurable range of 16 g’s. Or maybe it’s set to report data in 20 bit mode, which has both the high precision and full range.
The only way to deal with vibration (besides making mechanical changes) is filtering, which offers the “numbing” you’re looking for, with the benefit of being able to target which frequencies are deafened. Nowadays Ardupilot has quite robust filtering features. Have you tried playing around with the excellent filter review tool?