So I found a t70 socket at harbor freight. . . I got both transmission and rear diff oils changed last night, and it made no difference - the sound persists despite all my efforts
On the upside, it does not seem to be a problem with gears, as both drain plug magnets were free of metal bits or shavings.
Is there some bearing or bushing that's specific to 4th/5th gears? I'm ruling out center diff, because I'd imagine if the noise were coming from an unhappy center diff, its characteristics would change when I'm turning.