03 Silverado axle swap

[CHEVY THUNDER 90]

Hello all! I am an active duty soldier just returning from Iraq to my baby I left for a year. My 2003 Chevy silverado SCSB z71 4x4 lifted 4inches on 35s (With carefully cranked t-bars and added leafs) Does anyone have any suggestions for fairly easy axle upgrades? It currently has the 8.6 posi 10 bolts with 3.73s. I know I need to address gear ratios and hoping to do that also. I am a tank mechanic with mechanical knowlege..just need ideas. Thanks for the help!