Rear Differential Question

[Madbomber88]

I'm from the 67-72 blazer forum...
I stripped the rear gears completely a few weekends ago and am deciding whether to buy a completely rebuilt axle, have mine rebuilt, or to go to a junk yard and gamble that I am getting a decent piece.

It is a 12 bolt rear with 3.42 ratio out of a 71 K5.
The reason for this post here...
My father has a 78 scottsdale 1/2 4x4 pickup sitting in his field for the past 10 years since the transmission went. I am pretty sure this is a 12 bolt as well. According to my small amount of research, this may be an even swap. My blazer came from the factory with a Posi, is it possible the truck axle has one too?

I'm looking for opinions and advice here. It's $700 to have mine rebuilt (more if the posi is shot), $1295 to buy a completely rebuilt one, and free for the other one, if it will work and if it is in decent shape.
Thanks in advance...
-Tim