kirk280zxv8

Its easier if remove the differnetial out of another car and swap into your car. The best way to fix your issue of wanting to lower the rmp's wile crusing is to put in a transmission with a overdrive gear. The 700r4 works well and many people have had good luck with in them in many differnt applications. With a taller gear ratio in the differntial will only lower your rpm at highway speeds about 400-600 rpm and in the prosess you will loose the acceleration advantages of the lower gear ratio.

What are your plans with the car?

Ive got one already but it never hurts to have a backup. How much are you looking to get rid of it for?