Have a 1990 skyline wagon with a noisy diff.

I am looking for the setup specifications for the diff eg backlash etc. and instructions or a manual that contains them.

I have a gregory's manual but it has very little in it in regards to Differential.

If I do need to get a new crownwheel and pinion does any one know of a quality supplier.

I am a little worried as a friend ended up putting 3 diffs in his skyline before he finally sold it. Any help or constructive suggestions would be really appreciated as I am organsiing the car for a daughter and the rest of the car was in pretty good nic.