Installing a lift doesn't void your warranty from what I understand, but they can flag your vehicle (which means you might have to jump through some hoops for service). Ultimately they have to be able to prove that the life was the direct cause of the defect if something did happen.
I believe that this is true. There are one or two other folks on this board that can probably shed some extra light on this if they choose to.
But here's something else to bear in mind - if you go to the wrong dealer, or get the wrong service writer, or the service manager is having a bad day, or you didn't clean the dirt off the bottom of your rig and the mechanic had a fight with his wife that morning, you're going to get a lot of grief over asking for any sort of warranty service on a vehicle that is modified and has been off-roaded.
If you go to the right dealership, get the right service writer, and the phase of the moon is correct, you'll get whatever has gone wrong covered under warranty with no issues at all, provided that it's not caused directly by the modifications that you put on, or by you abusing the jeep.
I have experienced service at local dealerships at both ends of the spectrum. I walked out of one dealership wanting to wash the slime off of my hands after talking to the service manager (they ended up firing him), and got $3000 worth of warranty work done to my rig (lifted, on 35's) at a different local dealer and they didn't have any issue at all with the modifications because they didn't cause the issue.
Your mileage may vary.