The springs are a good indicator that you have an aftermarket posi unit. The Gov-Loc offered by GM had been around for many years and it is very possible your truck left the factory with e Gov-Loc installed. That is why it indicates the locking diff on your spec sheet.
And as our trucks are getting older, the likelihood of finding one that the factory Gov-Loc has been replaced is only going to grow higher as more time passes. Even with trucks equipped with open diffs per the spec sheet warrant a check to see what you really have. Plus it's never a bad idea to do a complete powertrain fluid change on that "new-to-you" square body truck that just followed you home anyway.
Posi/limited slip units require the limited slip additive.
Open diffs and to my knowledge "lockers" do not.
The reason is that a Posi unit has for lack of a better term, a clutch of sorts inside. The additive is for protection of the clutch discs.
Open diffs have no such clutches to be concerned with.
Lockers have no such clutches either. They lock the diff mechanically. (as I said, to the best of my knowledge).
Being that the Gov-Loc works mechanically as well, I do not see why it would need limited slip additive either.
But by all means, if you end up getting yourself an aftermarket posi or locker, seek the advice of the manufacturer of whichever you end up with to determine the best lube and/or limited slip additive.
Here is a pic of a Gov-Loc unit as you would find it when you remove the diff cover. Just remember the window may not be facing you when you remove the cover, so rotate the axles as needed to get a full view of what you have for a diff.
It doesn't take much looking around in Google to find alot of bad things said about the Gov-Loc. But if you find out you have one, draw your own conclusions on that subject.
