i don't know what year your jeep is or what model, but rubicons come 3.73 or 4.10. all the others are 3.21 or 3.73.
costs to regear is anywhere from 1000-1500 depending on location.
A lower gear "4.10" for example will be able to run 35' tires without the jeep being a dog. A taller gear "3.21" is good for fuel economy but limited to 33's from a preferred standpoint.
at the end, it will be based on preference. It is NOT needed, but very desirable once you upsize your tires.
most people that don't have rubi's tend to get lockers as well when they regear.