It may me a much simpler problem.
The "Traction-Loc" (posi) clutches will sometimes "groan" when turning at low speed. You might want to try changing the gear
oil and adding the proper amount (4 ozs.?) of "Friction Modifier" that is required for the Ford T-L differentials to keep the clutches quiet. You can get it at Ford/Mercury dealers.
It has cured the noise on three Fords I've owned. You may have to drive a couple of hundred miles before it quiets down.