There isn't any one mod that you can do that will void your entire warranty on your truck. I forget exactly what the wording in the warranty manual is, but it's essentially that if you put aftermarket parts on they won't be covered by warranty (obviously). Now that being said if you put an aftermarket part on that causes premature failure of another part your dealership probably won't cover that (such as balljoints, or CV shafts when doing a leveling kit).
So I say go ahead and do the leveling kit. 75% of the people on this forum have modified their suspension in one way or another and I haven't seen any threads with people saying they've had problems with their dealership on warranty work because of it. But like another poster said, it's never a bad idea to give your service department a call and see what they have to say about it.