For a little more torque, get a LCE header. Good bang for the buck. It's equal length and helps at low RPM.
To shift the powerband down altogether, regear. See below.
Originally Posted by afd23a
For 2005+ 5-lug trucks, the automatic transmission trucks have 3.58 and the manual transmission trucks have 3.31 gearing in the rear end. The next higher ratio available is 3.73 which is what comes in the 2005+ V6 trucks. Then there are 4.10 gears which come in the 2005+ 4wd/Prerunner 4 cylinder trucks. 4.56 and 4.88 are available from aftermarket retailers like East Coast Gear Supply or Randy's Ring and Pinion. Dakota Digital makes an electronic box that adjusts the speedo to be correct, although I'm not sure if it works for the odometer.
I regeared my 5-lug manual to 4.10's after lifting and larger tires. I replaced the speedo gear with one from a 4 banger 4x4/prerunner to correct the speedo and odometer. I don't know if you can do that with automatics. I haven't had any adverse affects and it's been about 2 years since I did the swap.