Rear differential question

[RodnRudy]

I put a '71 12 bolt rear in my 68 mainly for the 5 lug but it also had been changed from a 3-73 to a 3-07 which is what I was looking for. I've driven it about 3500 miles and the rear is real clunky when I put it in gear. Idle is set around 500 RPM. I'm thinking that the ring and pinion is incorrect for the carrier, three verses four. Now for my question. How do I tell if I have a three or a four series carrier in the rear? I'm suspecting that it has a four series carrier and who ever set it up couldn't set the back lash correctly. Either way it looks like I'm going to have to find a shop that can set it up correctly.