Honestly, they're not wrong. It's a modification of the drivetrain system. They have to be able to prove the modification caused the failure. It should not affect the warranty on unrelated systems, such as your radio. This is covered under the Magnuson-Moss Warranty Act, which is federal law. I recommend reading it. With that said, if your dealer is saying that up front I recommend a) finding another dealer that isn't such a tightwad, or b) get familiar with fixing your vehicle yourself or paying for somebody else to do it. I recommend option "b". At some point your warranty is going to expire anyways. I'm at 50,000 miles and almost complete with month 36 of ownership. My bumper to bumper warranty of 3 year/36,000 miles ended 14,000 miles ago. In 10,000 miles my 5 year/60,000 miles powertrain warranty will expire. It's just a matter of what you're comfortable with. With that said, these trucks are built to tow 9,000+ lbs. I don't think some spacers and slightly bigger tires are going to mess them up unless you do something to really mess your truck up.