There is the high idle written into the trucks strategy, which will auto idle up after a certain amount of idle time and if the oil temp is below a certain number. Typically the highest it will idle up to is 1000rpms.
Then of course you can do the "high idle mod" which is wired up through your upfitters. (If you don't have upfitters factory, you can add them in the 05-07 trucks). You have to be in Park, and have to have the e-brake kicked on, then flip up the upfitter you wired it up to, and it will force a 1200rpm idle, and it will also lock the converter to aid in warm up. There are two ways to do the forced high idle (different wiring) one way does it like this, the other is meant as a battery charging device, it will idle between 1200-2400rpm's to maintain juice.