So what's my next step?

I agree, you should go with the 4.10s. The next step after that is just to continue all of the bolt on aftermarket parts. I'd definitely do your exhaust next. Upgrade to 2.5" h-pipe, and catback system with better flowing mufflers (DynoMax, Bassani, Borla, Flowmaster, etc.). You are going to want to get some headers too. Then a bigger Mass-Air meter, throttle body, EGR spacer & maybe some underdrive pullies. You'll be shocked at the differences all of these mods will make once they are all on your car. Gears will be the best bang for the buck though...by far.