Hi there,
I had this same problem with my GTR but I actually noticed a difference in the car's behaviour first. In the boot under the parcel shelf there is a computer with an LED that flashes. It will just keep cycling through so next time you start the car and the light comes on, pop the boot and get yourself in there so that you can see the led. Count the number of flashes. If it pauses and then just flashes the same number of times again, then you're done. Refer the code to a manual or you can take it to a workshop and have them refer it for you. If there is more than one error code, the first one will flash, short pause, then the next one and so on. For example, my computer flashes 9 times, short pause, 8 times, short pause, 18 times, long pause and then repeats the cycle.
Have you checked the fluid level in the reserviour in the boot? This is behind the trim on the right between the back of the wheel arch and the tail light. Hopefully the error codes from the computer will give you your answer, because there are so many different things it could be. If the fluid level is fine then I would bleed the system (nipple at back of the transfer case and above the diff) to make sure there isn't any air. Check that your tyre sizes & profiles are the same, as different sizes can damage the diff. The important thing to remember with this in mind is the rolling diameter must be the same.
To be completely honest with you, having been what I have been through, I would just send it to a workshop. But if you want to, you may as well check as much as you can yourself first. Check the sensors underneath your centre console, if the indicator is white then they should be fine, if it's red then it's probably a wise idea to replace them. When was your front diff oil last changed? The next things that I know of are worn clutch packs in the transfer case or a pump that is on the way out.
Best of luck with it. And on a finishing note i'd like to say thank you to Duncan and GTRKen from these forums for your help and patience with my attessa questions and problems. I'd also like to thank my workshop of choice for diagnosing the problem when i'd had enough of trying to figure it out for myself (Speedworks) haha.