That's really odd. Your '81 K20 should have come with a 14 bolt FF from the factory. And, even if a 10 bolt was swapped in for some reason, they never came with 8 lug axleshafts. In short: I guess I don't really know the answer to your question /forums/images/graemlins/smirk.gif /forums/images/graemlins/confused.gif /forums/images/graemlins/crazy.gif
Wait, on second thought....I believe the Dana 60 rears have 10 bolts on the diff covers. Two questions for you: 1) are there big hubs coming out of the center of the wheels 2) is there a fill plug in the diff cover? If so, then I'm gonna say you have a Dana 60. I don't know why it's there, but I believe that's what it would be.