Can you identify what rear diff you have by the numbers on the housing?? I have a 75 280

Easier to just look at the rear cover, or the area around where the axles insert.  The R200 has a rectangular chunk of aluminum, about 1/2" by 1", underneath the drain plug and the cover is almost round.  The R180 is smaller and has a rectangular rear cover.  It also has a ring of bolts around the axles holes, where the R200 does not.  Look at some images on the internet and you should be able to tell with a glance.

If your 75 is a manual transmission coupe it's almost certainly an R200.

Or you could post a picture and we'll just tell you what it is.

Thanks,  I need to run down to my shop and look at the housing and take a picture while I'm there should I pull the cover off the pumpkin in order to tell if it's LSD or is that not necessary I'm assuming it probably is not

It is probably not unless someone swapped it. If you jack the rear of the car up and turn one wheel, if the other rear wheel turns in the same direction = LSD if it turns in the opposite direction = open diff

Here's an old picture I had of an R200.  You can just see the rectangular piece of aluminum molded in to the bottom of the cover.  And you can see the rounded corners.

I counted 39 teeth

Somewhere on the next to the ring gear you'll find 39 and another number stamped beside it.  Like 39:11.  Looks like a run-of-the-mill open R200 with 3.54 ratio.  Could also be a 3.9, from a later model 280ZX.

If you're not sure which ratio R200 you have lying around the garage, check it this way: Remove the rear plate (drain the oil of course). Then rotate the large ring gear until you see these numbers stamped into the edge facing you. 

• If it's a 3.54 it will read "39:11"
• If it's a 3.70 it will read "37:10"
• If it's a 3.90 it will read "39:10"
• If it's a 4.11 it will read "37:9"

Great information, thankyou all.