hummmm...
centre diff borked?
I'm not certain from memory, but I'm pretty sure the GTiR and the U12/U13 SR20DET powered bluebirds have the same rear diff/transfer case ratio. However, I maybe wrong and fitting the GTiR gearbox with the U13 transfer case may fix the problem (front/rear final drive ratio is inside the front transfer case and rear diff, they MUST match).