It looks like most folks with this problem have the 3.73 gears. There is some resonance in the driveline at certain frequencies and having 3.73 gears will lower the speed a certain harmonic will be reached.
Transmission tailshaft-driveshaft-pinion all have some variation and the lengths and all really cause it.
The new Cobra with Independent Rear Suspension had major problems with this.
Hope this helps,
birdman

 

Had the aux. tranny cooler replaced today and didn't hear a peep from the drivetrain as I drove it home from the dealer. I didn't drive it for very long, but there was no trace of the "moan" or whine at any speed between 10 and 35 mph

So for all those waiting for my "report", go get it done

Jim

P.S. Superchip "Flipchip" is now on order -- thanks Mike!