You can do it with just shackles but that's not the best way to achieve the tire clearance. Shackles adversely affect the caster and pinion angles, and decrease approach and departure angles.
You will need 2" of lift to clear 31" tires, which means the new shackles would need to be 4" longer than stock (shackles only lift half the difference in height due to placement at only one end of the spring).
Consider a 1" body lift and installing TJ fender flares, will easily clear 31" tires. A 1" hockey puck body lift can be done for $25.