G70 -- or any lube -- won't fix a grinding sound. It can reduce a whining sound. To fix grinding, you have to replace parts. It could just be the brake parts rubbing.

I wouldn't rebuild any diff when good used units are only $150 from the wreckers. Too many home rebuilds don't last, and if you pay a true expert to rebuild them it is $$$.

Best ratio for an A15 is 4.11. Nissan fitted 3.70 to them for fuel economy, not because it suited the engine any better. With the A15 they could use 3.70 and still have the same slow acceleration as an A12 with 4.11 gears, but it is not performance-oriented. As a compromise use 3.90.