Mystery drivetrain noise help!

[Korr053]

Hi, I have a 71' C30 dually that has me and all my shadetree buddies stumped.
The problem is when driving I'm getting a terrible howl/grinding/scream from somewhere in the drivetrain. (I think) The noise isn't bad in low gears but when I get into fourth and get going 45-55 it screams. When I press in the clutch it stops. I replaced the tranny, a sm420, and the clutch with no luck. Dosn't sound like the rearend, any ideas? Thanks!