It's going to be so close to level on your '04 that it won't be worth worrying about whatever difference there might be afterward. You're going to love it.
04-07 need 2.5"; 08-11 need 2" (I have measured it out and with a 1.5" leveling kit I need another 1/2" to get it leveled) and i heard that the 2" wont make any contact on anything
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