i have a 2013 and i plan on calling a soon as i get a chance and verify that very same question !
normally i wouldnt consider changing exhaust that is brand new but it is just to freakin quiet and it is in fact a pickup truck not a luxury car.
It shouldn't. The only way they can void the warranty is if the exhaust is the direct reason of something breaking and causing the warranty to be needed... BUT some stealerships are very picky.. So I'll stick with my answer that it shouldn't... Definitely call / get credible information before doing anything.