The sensor could indeed be a red herring.
I run with what I read and that gets me in trouble sometimes. Just making sure we don't suggest the wrong splines. I am curious if the 280zx turbo shafts are thicker to make up the slack in the J30 center. I had assumed they were the same since people used to jam the 280zx turbo axles to get CV axles in their 280z's without swapping differentials. I found it really tight to get the axle in there. Are you planning a swap into a long nose? I would be curious to see what you do for your axles. I have to revisit mine and I'm thinking the AMC axle route.
For the domestic market I think the 3.9 is more or less limited to the 300zx and maybe one or two years of the 280zx. I know the R30, C31 turbo I think both came with 3.9 as options, that is with the assumption that the ring gear wasn't just swapped onto an lsd center section. Being in canada the variety is up there to be from something we really are not sure of down here.
It looks VLSD to me, my open 300zx looked like a traditional open R200 from a 280z. The full housing kind of points towards the VLSD nature, and the clutch types you can usually see discs for maintenance purposes through the view hole at least in the subaru R180's you can. It would definitely help if you could take a shot straight through the center from the axle point of view, we can count splines and determine if it has the spacing for the VLSD center which would narrow the search.
I think more or less the bottom line depends on some assumption or eventual deductions. If it is a 29 spline center VLSD, your best bet is to find a non TC or ABS (I can't remember the one without the blades on the outer shaft) J30 and pull the input shafts from that, then run a hybrid axle linked above if you were so inclined to use the differential. If it is a 30 spline center VLSD, a Q45 is probably the car to pull the input shafts from and adapt your way out.
What makes you think M30 specifically?