[MJN]

As it was stated above the best you can do for that rear axle is 4.10's. Standard gear ratio for these was 4.56- thats most likely what you have. If you want to confirm there are numbers stamped on a flat area of the third member above the pinion or you can pull the cover and count the gear teeth.
Besides an overdrive unit or 5-speed transmission is to swap the rear axle out and that would either require fabrication to make a 14-bolt (they can be found with 3.73's) work with your coil springs or trying to find a Dana 60 from a coil spring truck that would bolt in and having a different gear ratio ring and pinion installed to get the ratio you want.