Don't worry, Glo-
With the azipod propulsion, there is virtually no noise or vibration, and due to the righting moment of the ship, the aft cabin motion is very minimal--not at all like the front of the ship. On the inside passage, there is very little rough water anyway, so it's almost a moot point. You are going to have a great time, and you have the "best seat in the house!" We've had a (very)forward cabin on the Mercury and both aft and midships on the Infinity. The only one where we felt any motion is in the forward one. No noticible difference in sea motion between the midships and aft cabin.
And Gunner-
The aft cabins are popular because they are larger, they have the largest balconies, and the view is so good. You really lucked out!