Let me categorically state, that I absolutely hate stupid, idiotic -"we can't do anything because we might get sued"- tire shops...
I started working in the tire industry in 1976. I know of what I speak.
1- Stick with the 17s. A taller tire sidewall will yield a better, more compliant off-road ride, not to mention more tire choices.
2- You should have -0- issues running up to 285/70R17s. With a 2" level, I am running the equivalent of 34x12R17 (305/70R17), and no rubbing issues. (I can say this, because just 3 days ago I did Mengel Pass and Steel Pass in Death Valley - both "Class IV" Jeep trails). And that, in a 145" wheel base truck! (you should have seen the looks on the Jeepers' faces!
3- levelling will NOT (I repeat: NOT) cause warranty issues. Levelling will not cause some other vehicle function to fail; thus, no warranty issue. I levelled my truck at about 10K. All my dealer's service mgr. says is "nice set up". All my warranty work gets done with -0- issues. I now have 48K.
If you act naïve, they will treat you that way (because they know they can get away with it): don't let 'em!