i dont know if buddy club would sell the lower shock housing... it would be unlikely I'd say.
I thought of something else yesterday - I don't know if the GTS4 rear end has the same alloy hubs as the GTR but I think (and hopefully someone can confirm) that it is the same fitting fork fitting as the GTR.
It would surprise me if nissan went to the effort of engineering a cast iron rear hub specific for GTS4 with a fork mount - but stranger things have happened.
Another thought - it might not be a Skyline rear end at all? I don't know why or if anyone could/would swap it for a 180sx or similar rear end, nor the mounting points of any other nissan rear ends. I know people swap skyline rear ends into silvias...
Sydneykid has also made comment elsewhere with Nissan doing a few 'parts bin specials' on the late cars, so you never know your car may have come out with a different rear end from factory...
Glad to add to your confusion!