help with rear end gears please

[hitcher]

hi guys

ok i have a 88 1 ton crew cab 4x4 .at 55 mph she is screaming 2,500 rpm which wont do . i have 4:10 gears in the rear and want to change them to get the best fuel economy that i can . now im asuming the front differential is the same and i need to change that along with the rear ? never done anything like this before so im reserching it first .
so whats the best gears i need for fuel economy and is there anything else you need to know to help me out ?

thanks