A lot of people will quote the Maggnuson/Moss (spelling?) warranty act, which basically says that the dealer has to prove an aftermarket part caused a failure. However, dealers have a lot of say in what is and what isn't covered. You have some dealers selling new trucks with full suspension lifts and superchargers and some that threaten to void your warranty if you go up one tire size. When I purchased my Wrangler, my dealer agreed to buy and install my lift kit as part of the deal as well as buy back my OEM tires/wheels so I could install larger aftermarket items. Best advise is to ask YOUR dealer. If you don't like the answer, tell them you are going to take your business elsewhere.
But the whole idea of the lift has to be a Mopar part and they have to install it is complete BS. If that was true dealers could void your warranty if you do your own oil change and use a Puralator or K&N oil filte!