Ok, gotcha. I was thinking the truck was the
GMT K2XX, but it's a GMT900...yeah, I think the 8.5/8.6 are basically the same, with appropriate bearing kit per that article I linked.
You got to setup the backlash, etc., right? I know the G80 has a bad reputation, but in stock form it works well. My 99 Silverado 2wd has one, and it's saved me more than a few times from getting stuck. I have LT265/75/16 tires on it.
Any G80 is better than an open diff when you get stuck. And free is even better in your case, LOL.