One of the engine
I'm using an S15 diff inside an S12 case. I have 2 sets of 280zx driveshafts. Now if I was using an S13 VLSD I could use the 2 short 280zx shafts in exactly the same way as you would with an S12.
But, with the S15 diff you have to use both the S15 inner shafts (not one like you can with the S13 diff), so whilst I can make one up with the shorter shaft, I will need to get a special short one made.