Who. Better to work on an lsd rear? Mechanic or transmission guy. I know this mechanic I trust very well prices are good. Transmission guy was more. Who is best suited?

 

Of the two your choice is as good as anyone else's or find a drive train shop. Drive train shops sometimes include trans and rear diffs.

 

Like a four wheel drive shop?

 

what is the work to do on the diff ?
If you can R&R the diff Nissan dealer probably the place IMO
as the cost of overhaul between them and a general shop, good well known trans shop is not a lot of difference I have seen it cheaper.
there is a spreader tool to get the ring gear assy out and usually can be a specialty tool ( only nissan is going to have ) does the private shop have it ?
and when they say "oh yeah we got the tools", ask to see it my guess is "they can't find it right now"
mostly you want a auto technician that has done the repair you need done more than once ,twice and not many years ago which for a z31 could be hard to find

 

Have to get lsd first from Canada then see, got the gasket, need finned cover if anybody has one!