Jim,
Hate to say it, but from your brief description, it sounds like you either have a bad pinion bearing, or you may have lost all carrier bearing pre-load in your diff, allowing your gears to come out of mesh under acceleration. Either way, I recommend taking the time to dis-mantle and inspect it before it gernades.
I have seen a bad pinion angle cause a "howl" before, but it was only under extremely hard acceleration.(Always accompanied by a vibration)
Has the noise been gradually getting worse, or did it start all of a sudden?
Have you done any work to the springs or diff/ drive shaft recently?
Can you make the noise increase or decrease by weaving as you drive?