What rear diff do I have? '86 K20

[VTKidder]

I just bought a plow truck that has an open rear end, and would like to put a posi unit in instead. I have not checked to see what rear end I have, but was there one that was standard on 4x4 3/4 tons? The ratio is 4.10, and I believe it is an 8-lug truck. Are all 3/4 ton 8-lug? If a truck is equipped with a full floater rear end, A) would it show on the RPO tag in the glovebox? B) would a "dog dish" factory hubcap fit, or would it need to have the hole in the middle like a front differential hubcap has?

Just looking for some help on what I should keep my eyes peeled for. Thanks.

-adam