YOu can get a 3.42 rear axle ratio, fully assembled, and ready to go.
There are many vendors that have them, however, you may want to check here, before searching.
GM does not allow these axles to be rebuilt, as there is no special tools to correctly set side carrier preload.
YOur best reliability will be to purchase the whole differential.
As usual, c4c5 is right, you will have to replace the whole unit. The first c5 diff i pulled out I had intentions of rebuilding it. After researching it I discovered that GM does not want you rebuilding them. If I were to switch ratios I would get a whole unit, I wouldnt trust one with just the r+p changed.