Sounded to me like your starter was making some noise when you first engaged it. Like the return spring wasn't returning the starter gear properly. If the starter is still spinning with the engine that would whine A LOT! So I'd check that first.
If it goes away when the shifter is moved around it has something to do with the trans I'd think either in the linkage or a worn transmission. Toss someone in the bed and see what they hear in the back! Diagnosis from the cab sucks since there is so much engine noise in these old trucks.
Jeremy
