The rear is where you want to go if you do just one.
That way, you have a locker available on your main drive wheels. That's where you need the most traction. Also, unless you do a selectable locker, you'll alter your Jeep's steering with a lunchbox or automatic locker up front.
With the larger number of selectable lockers out there these days, they're not that much more expensive.
You can pick up the Auburn ECTED for about $650 for the Dana 44 (3.73 and down carrier break ONLY for the JK, regardless of gearing!!!!), which is only about $150 more than a Detroit Locker.
Doing front and rear is obviously a huge improvement, but a lot more $$. Installation will usually run $300+ per axle, so keep that in mind.
Also, if you are swapping in lockers, consider your ultimate tire size goal, and re-gear for that now. You shouldn't be charged any more to re-gear now, as the installer has to remove the ring gear anyway to install the locker.
I see you're out west. Due to you being in rocks, I'm not AS big on air lockers as I would if you were in mud-only environments. The air lockers are GREAT, but they are rather vulnerable to snags. They can also be finicky. The ECTED is my choice due to the fact that you can make trailside repairs to an electrical wire- an air line, not so much. Also, with air lockers, you MUST have a compressor to run it.
Just some food for thought... post up any further questions