I ran 305/55r20s on a 1.4" lift on my 2015 Ram Quad Cab. It's not really a lift at all when it comes to clearance issues. In fact a coworker had a similar Ram that sat at virtually the same height with no lift at all. My wheels had 3/4" less back spacing though, which means they stuck out that much more. That really should only affect clearance of the upper ball joint which is pretty close. I bet the dealer would test fit a tire to see how close everything is.
Unless you're talking about a taller tire. I think people routinely run 275/65r20 on stock rims with no lift. That's about an inch taller. Dunno if you can go more than that or not. Once again a test fit might tell you all you need to know.