I had the exact same problem on my GTR. I never really understood what the problem was but believe it had to do something with the rear wheels being stuck at the 2 degree angle (whaterver the HICAS does in the back) to compensate the car naturally turns the front wheel as well to go straight, here is a crappy illustration:
/ / front wheels
/ / back wheels
So in theory the car should be going straight down the hwy at an angle - I could never prove it, seems stupid actually.
Anyway things I tried to do to fix it:
wheel alignment, HICAS reset procedure, new tyres, all which worked to some effect but the problem always came back.
don't replace the steering angle sensor, this does not fix the problem.
I later replaced the rear and front bushes (whiteline) and never saw the problem again. The comment about the rear rack in my opinion is correct.