i have a similar decision as we speak. i just dropped front differential today with 370 ratio because of pinion shaft bearing play. i had to decide if i wanted to pull rear diff too and go to 355's or 358's; or only repair/rebuild front diff. i decided to stay with a 370 ratio because at 60 mph and slower, the truck will get better fuel mileage. and besides the truck has 11.1 detroit 365 hp, so will do a little better pulling. i have a dedicated situation which i almost always stay at 60 mph and below, but sometimes i have to pick up the pace or sacrifice what little free time i have. i'm not sure exactly what the fuel mileage drop would have been had i went with 358's and drove 62-63 mph and rpm's slightly below running 60 mph with 370's; but if i am right, most of a truck's fuel mileage loss is from aerodynamics, not higher rpm's. however, if you know your going to run 65 mph, i would make the switch to the used diff's because the price is the same and not many miles on them. besides,if you doing the front diff now, i am guessing the rear diff has it's wear.