First, some 280ZX's came with the R180, they all didn't have the R200.
The differences between the Subie Diffy is that it uses clip-in flanges, so you would have to grab the axleshafts from something like a 200SX, and some companion flanges from a Turbo ZX to make a compatible CV-System.
This is not too hard, in LA you can get the R180 stubs from the SX put onto the ZX Turbo Axle shafts for about $50 including new boots.
The other thing to be wary of is that the normal XRX does NOT have an R180! It is the STi model that has the R180 LSD. Now, if you are in Japan, or getting JDM parts imported it's different, but in the US market, the STi is the only one that has the R180 diffy out back.
So yeah, it can be done, but by upgrading to the CV joints, you might be as well off swapping your rear differential support to the R200 configuration, and installing the R200 LSD---they are available, and will hold up better if you crank out some serious Turbo HP...
As with anything, everything can be accomplished, it just takes some fenagling sometimes.
And always remember to carry a tube of aluma-seal in your glovebox. You never know when that headgasket's going to go!
just make sure the subie comes with the axles or at least the stubs.
You can get 280ZXT axles and companion flanges easily enough through the mail, and any engineering works should make quick work of the tripod rebuild using the subie stubs to the ZX axles.
You will have a quieter, more vibration free car without those u-joints out back, too.
Without the stubs from the subie, I don't think there will be a way to adapt it to your application.