The '76 Blazer I have also has a 12 bolt under it, and it is about to get an Auburn, new 3.42 gears and all new bearings. I, too, really need the posi most when lauching/pulling the boat in/out of the water. But it is a nice reassurance to know that if a situation comes up, both tires are going to pull
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As far as I.D.ing your rearend type, just lay under the rear and count the number of bolts holding the rearend inspection cover to the housing. It will either be 12 or 10, matter of fact, if there is only one bolt at 6 o'clock it's a 10 bolt, but if there are 2 bolts parallel at the bottom of the cover it's a 12 bolt. That will at least get you on course for knowing which rearend you actually have.