The best that I can tell is the OEM spring length is the reason behind the different size spacers to get level. It's not an exact science but it gets the front end close to level.
04-07 4x2 uses a 13.2" spring and a 2.5" level kit
04-07 4x4 uses a 14" spring and a 2.0" level kit
08+ 4x4 uses a 15" spring and a 1.5" level kit