When I did the poly's I did both. I see no more vibration in the scoop or intercooler. Also it would be wise to put the stock tranny rubber mount on and not use the poly mount. Keeping vibrations out of the transmission is beneficial. Plus that will help counter the stiff motor mounts. Some people are very sensitive to the vibrations so they refuse to do both mounts. When you make more power, you want assurance you won't rip the mounts apart and break something. I don't know exactly, but my logic would tell me that one mount is rubber and the other is solid. So now the vibrations aren't symmetrical causing irregularities and giving you a "violent" shake. I'd say change the other poly mount. If the "violent" shakes go away, they were absorbed into the body and frame. If they don't go away go back to the passenger rubber mount.