You can put the lift on and it isn't going to completely void your warranty. Don't listen to dealers on warranty issues, as most don't fully understand how it works when it comes to aftermarket parts.
For example, if you install the lift and have U-joints go bad or suspension related issues they may not warranty it because it was a result of the lift. Some dealers may be a little more friendly and take care of it anyway.
If your window regulator goes, or a CKP sensor or something of that nature, they can't deny warranty based on your lift. Some dealers use modifications as a way to say that you must abuse your truck and the failure is a result of customer abuse. It is in their best interest to perform the work as they are making some money on it but if they have gotten audited in the past from OEMs they may be hesitant to blur the line a bit. Pair that with a gross misunderstanding of warranty, and they aren't going to want to touch your truck.
Orchard Chrysler/Dodge/Ram/Jeep up in Washington is pretty mod friendly.