The easiest way to measure backspace is to lay the wheel face down onto the ground so the backside of the wheel is facing up. Take a straight edge and lay it diagonally across the inboard flange of the wheel. Take a tape measure and measure the distance from where the straight edge contacts the inboard flange to the hub mounting pad of the wheel. This measurement is backspace.
So, the less you have, the closer to the steering and tub of the Jeep the tires and with wider tires, you may get some rubbing while turning. You can overcome that by changing the length of the stops (if you look inside your tire when they are turned all the way, where the wheel "stops" is a stud on your frame).