I know I'm biased...but even if you don't run synthetic oil in your engine you should run synthetic lube in your differential regardless of tranny type.

1) It takes less than two quarts. So cost of product shouldn't be a huge issue.

2) With a good synthetic such as Amsoil Series 2000, the lube will last the life of the car (500,000 miles) easy. I change at 50,000-100,000 miles because I get the stuff wholesale and my wife loves the smell...but seriously there is no need to replace unless it gets contaminated with water, etc

3) It will make your rear end last much longer.

Use 75W-90 or 75W-140 (my favorite)