The noise you have in the diff does not necessarily mean the backlash is out. But it does mean something was out last it was adjusted. And the noise sort of means its too late. You can adjust it right now, but the noise will probably always be there, untill you buy a new crown wheel and pinion. The diff will probably last a long time with that noise, its just annoying on the highway.
You will notice the noise is not as bad in the morning, when the diff oil is cold and thick. Thicker oil will make it quiter a little, but only untill it warms up, and a diff does get fairly warm.
I have tried the bananna peal also, and yes it may work a little.
Don't waste money on expensive diff oil, especialy if your diff is already stuffed. A standard car will survive 30 years without a diff oil change.
