If you change the rear axle you also have to change out your hubs to the 04+ which are also lighter. The pre 04 hubs will not fit the 04+ axle.

Focus on reducing rotating mass (axle, hubs, 520 chain/sprocket, tires/wheels) as they will give you the biggest bang for the buck.

 

My hubs are getting turned down this week.
Are the spline different on the 04+ axles? My axle has a steel chain sprocket hub. I guess the newer ones have an aluminum hub. Do you know if the hubs are interchangeable? I have to look into 520 conversion. Any idea how much weight that saves?