We just got off of a 30 day cruise on the Voyager in the aft cabins (category C across the very rear end of the ship). We LOVE these cabins, and we always try to book them, but lots of folks don't like this location. Yes, there is some vibration at this location, but we kind of like it - just like we like the pitch and roll of rough seas! Not everyone's cup of tea, but we find it helps "rock" us to sleep!
We did have to do some "adjustments" during the cruise (putting padding a few places in the walls/ceilings and rearranging bottles/glasses), but we are used to this for these cabins, and find it worth it (these aft cabins have awesome large decks).
If you are bothered at all by vibration, I would recommend going more mid-ship.