The new gears give you back the power you lose by switching to a larger tire. When you switch to larger tires, you alter the final drive ratio between your engine and wheels. Or in other words with larger tires, the engine will be turning lower RPM's at a given road speed. While it may sound like a good idea to have the engine turning slower, it actually will take more energy (and fuel) to run that way. It's similar to when you ride a bicyle in the highest gear at a low speed, you don't pedal as fast, but you must push a lot harder. You change the gear ratio in the axle in order to bring the engine speed back into its "power band" on the highway. By switching to a 4.10:1 gear ratio with his 35" tires, Sbveenker has returned his final drive ratio almost exactly to where it was with the stock 3.55:1 gears and 30" tires.
"Lockers" are traction aid. I'm sure you've seen (or had it happen to you) when one wheel on an axle loses traction and begins to spin while the other one stays motionless. This is what happens on a normal "open" differential because the power follows the path of least resistance (the spinning wheel). A locker will prevent this by locking the wheels together. This gives you much better capabilities off road because, as long as at least one wheel has traction, the vehicle can continue to move. The down side is that while "locked" both wheel must turn at the same speed, even in a turn. This can cause driveability issues on dry pavement.
People who drive their vehicles on the highway usually choose some type of selectable locker or one that will automaticly unlock when nesesary (like Sbveenker's lock-rite). For a trail only rig, a less expensive option is to install a "spool" which permantly locks the wheels to gether. An even cheaper option is the "Lincoln" locker. So named after a popular welding machine maker. To make a Lincoln locker, you simply weld the spider gears together in your open differential so they can no longer rotate, thereby locking the axle halfs together.
