Follow-up: I compared what came out of the rear-end to some fresh lube. What was in there was definitely not 15w40. I'm 95% sure it was 75w90 or thereabouts based on viscosity. It was definitely NOT motor oil, as it had the "gear oil smell and sheen" that I associate with zinc-based additive packages. So that was a relief. And frankly, it looked darned good for being in there for 3 years (but only about 4000 miles according to the records).
As an experiment, I ran it for a week with some cheap generic 90w lube from Tractor Supply that was on sale, and then I drained it and refilled with 75w140 Mobil Delvac synthetic (the local dealer, who appears to be a complete doofus, kept giving me excuses as to why the Lucas stuff hadnt arrived. I cancelled the order and got the Delvac which the local tractor pullers swear by). Its noticably quieter now.
This lube may be a bit thicker than the optimal spec, but I've never heard of a diff hurt by running a thicker lube in hot areas. All of my driving will be in the heat, (and 90% of it through a desert in the middle of summer), so I'm happy to have the thicker stuff in there.
Thanks to all who provided comments and guidance!
"You can finally say you have enough horsepower when you leave two black streaks from corner to corner"
(Mark Donohue, famed TransAm driver)