Howl in 4th?

Noise in all gears BUT 4th, with 4th being direct, would be a tranny problem. Noise in 4th would suggest a differential problem. In 4th gear, there is no load on the tranny, you could actually take all the gears out except for the input gear and the 3-4 syncro assembly and drive the car. The big hint is you said the diff. was re-done.

Hope this helps.

Russ.