Are you sure it's the tranny?
Did you check the flex plate while the tranny was out?
Since it's only in forward, how far is the prop from the cutless/strut?
What type of shaft packing do you have?
Does it whine when tied to the dock
and under power?
And when did you first notice the whining?
If it is the tranny, you can use a touch stethoscope to listen to which part of the tranny it's coming from.