Are you sure it's the transmission? Could possibly be the rearend whining, especially at those speeds when you have less load. At those speeds does it whine when you step on the gas or when you let off the gas?
"Greater love hath no man than this, that a man lay down his life for his friends." -John 15:13
“People sleep peaceably in their beds at night only because rough men stand ready to do violence on their behalf.” -George Orwell