Without touching a single thing, 275/60/20 or 295/55/20 should fit without any trouble. The 285/60/20 you mentioned should fit too, but there aren't many options in that size. I've heard of guys running 305/55/20, 295/60/20, or 275/65/20 at stock height, but some have rubbed a little and some haven't. I think those sizes are so close that it depends on the particular truck and the particular brand/type of tire. For example, a 305/55/20 BFG AT KO2 might clear just fine, but a Nitto Trail Grappler in that same size might rub. If it were me, I'd try one of the last 3 sizes I mentioned, and do a bit of trimming or cranking if needed...and then decide on the amount of drop in the rear after I had the tires I wanted. And, of those sizes, I prefer the 295/60/20, because they're a nice balance of extra height and extra width. Just my $0.02....