I did some research on this GM "grape juice" a couple year ago.
Here's a synopsis of what I can share. Make sure to note the there are "spec" numbers, and there are "part" numbers. The spec is for the fluid; the part is for the packaging.
GM #9986115 is a spec for the blended product of GL-5 hypoid gear oil (of synthetic base stock) and some friction modifier.
Why FM in a "locker" rear end? Because there is a small actuating clutch that moves the locking pawl mechanism, and that clutch can "chatter" at times. The FM quiets the operation of that clutch. It will not be hurt if you don't use FM, but it can be anoying both with audible and physical nuisance locking.
GM apparently buys the hypoid oil in bulk, and the FM in bulk, and mixes them at the plant? (Best info I could find at the time). So, to make the "grape juice" availabe over the counter, they had to create a new spec (9986115) for the "blend" of oil/FM, and then make it available in a bottle for sales over the parts counter. (there were, at the time, both pint and quart bottles, so the part numbers you may see quoted will vary, depending upon container size).
The reality is that the "grape juice" diff fluid from GM is nothing special; it's a "synthetic" GL-5 with FM. That's all. It performs reasonably well, unless you consider that it's stupid-expensive compared to its competitors such as Mobil 1.
Here is a link to a conversation regarding some information straight from some tech guy at Eaton, the manufacturer of the G-80 locking units. In a nutshell, they used "Texaco" brand fluids for testing, but he clearly states that multiple grades and base stocks can be used, as long as they meet GL-5. Read where he calls out both dino and synthetic options for the GM fluids.
If you'll peruse down about 1/2 way, you'll see a topic for "automatic locking differential lubricants"
Here's the link:
http://www.swedishbricks.net/700900FAQ/Driveline1.html
Amsoil also has similar info in their TSB #DT-2004-07-01.
Hope that clears it up. Use any GL-5 you want; you'll be fine. Use "synthetic" if you want longer fluid intervals.