8.75 Value?

I want to keep the existing 3rd member i have, but I do have an extra one. I have no idea If it will bolt on or how to identify what it is.

It will bolt in. Look at the last three numbers on the case. It should be 741, 742 or 489. Also if you look at the ring gear it should have the ratio stamped on it. Look at the center of it and if you see a large hole with gears showing it's an open diff. If it's more solid or you can see springs through a hole it's a suregrip.