Diff questions
 

My question is if all LSD diffs are the same from S4 and S5. In other words, would it be bolt on if you were to get a S5 diff and install it into a S4. I just bought a GTU LSD and the guy who I bought it from doesnt know what year the car was. I have a 88 vert rx-7 so is there anything that needs to be done if they differ from one another?

All the NA diffs are interchangeable, so you can just drop your open diff and bolt up the new LSD.

If it's from an '88 GTU, it'll be a clutch type LSD. But if it's from an S5 GTU, it'll be viscous. I don't think there's any way to tell the difference from the outside.

So is there any way to find out from what series the diff is from?

From what I've seen, the outside looks the same. You would need to crack it open to see what kind of limited slip it is.

If the owner knows it was from a GTU, and not a GXL, then it's more likely a going to be a viscous LSD from an S5. '88 was the only year there was an S4 GTU, but the S5 GTU was made from '89-'91.