djlantis57,
The problem is manufacturers lie. A lot. It's common practice for tire manufacturers to be off by +/-1", and a few are +/-2". So, your 31" might fit, and someone else's might rub. That's why the manufacturer only claims to clear 30" tires.
That said, a 3" lift should be more than adequate for MOST 31" tires. Some will even fit with the OME 1.5" lift kit. I have 29" tires on my 4-door and don't need any more clearance or larger tires. If anything I need a locker. There's a guy around here on 29" tires in a samurai that can kick my butt all day long (in my SPOA'd 31" tire rig) because he's locked front and rear. And a locker (and $200 2" spacer lift) is WAY cheaper than new tires, and Calmini's kit. If you go Calmini (or 2" spacer lift) and experience some rubbing, it's either because your backspacing is wrong, and your rubbing the frame (which lifts won't really help with), or your rubbing a fender / bumper. Just do a little trimming. It's virtually free (if you have basic tools), keeps your center of gravity lower, and looks better anyway.