1991 5.0 mustang aod need help!!!!!!?

ok so i have a 1991 5.0 mustang and the overdrive does not work!! also when i switch it to 2 or 1 the car doesnt do anything? is it a easy fix or does it mean i need a new trans? also what gear ratio should i go with? 3.73 or 4.10? for a stock car?