The rear is the issue, if you put more backspace the tires will rub the springs, and they will tear up the the lip if you don't trim. It's actually not much and unless you have it pointed out not noticeable. I actually thought Marn had no trimming until it was pointed out. I've seen a few GW and Wags run 33x10.50s on 4" and they all have some trimming and work really well. I run 33x12.50 on 3" but I'm wide track so hard to compare. 31s should fit no problem without trimming though.
you wide track guys have it made. 33's and no lift works.