Honestly, I'll recommend upgrading to 4.10's for 33's. The overall performance will be better and you won't see any mileage decrease. But ultimately, it's your choice.
One option that may save you money is to surf craigslist to see if you can find someone selling stock axles with 3.73's already in them. It would be a bolt-in swap to change them over and they will most likely cost less than regearing.
I would stay away from used gears. No installer I know would warranty their work with used gears. The chances of getting them installed wrong is too great.
Also, gears are very specific to each axle. You can't use GM or Ford parts in the Dana axles under a Jeep. They have to be gears specifically for a front JK Dana 30 and a rear JK Dana 44. There's no way to make any other gears fit.
Beyond that, the installer you choose will be able to order all the right parts for you. But so you know that you're not getting ripped off, these parts will be in the list:
- 3.73 gear set for both the front and the back.
- master install kits for both the front and back. These have the bearings and shims needed to set up the gears correctly in the axle.
- One new differential carrier for the front axle. Going from 3.21 to any other gear (3.73 to 5.13) in the front axle requires a new carrier. You have the option to upgrade from an open differential to a locker of some kind and only have to pay the difference in price. Read up here if you don't know much about lockers: The Basics of Differentials and Lockers
- Gear oil
That's all that is absolutely necessary.
The rear axle uses one carrier for all gear ratios so you don't need to upgrade it if you don't want to. But it makes sense to do so when you do gears because you are already doing the work and you only have to pay for the part. There is no additional labor (aside from running the control for a selectable locker) if you upgrade while you're in there. Even if you're not going to go off road a lot, upgrading to a limited slip in the rear makes sense for on-road driving if you live where you get ice and/or snow.