I agree with RCMAN, to a point. Lower gears in the rear end won't make any more power. It will just change the road speed at which your transmission changes gears. That said, it will also decrease the spacing between all of your lower gears which will help with the "torque rise" another poster mentioned. If you're running in 6th gear where you used to run in 5th you now have 5 gears between your current speed and 0 instead of 4. If you don't need to shift on the hill, there will be no difference between a higher rear axle ratio and lower transmission gear versus the opposite.
The reason a gear swap won't matter nearly as much as in the old days is that the rear axle gear swap may move you from 4 to 5 gears to downshift into instead of 2 versus 3 on an old 4-speed. Even with 3.73's, the gear spacing may be close enough already and you just run one gear lower. It's about matching the ratio spread to the power curve of the engine which is why you wouldn't want 2.20's and 2nd gear.
