That sounds exactly like what happened to my '04 Rubicon's rear locker, it sprang a small pinhole leak inside its internal bladder that is used to lock it up. Replacing the bladder inside the locker is not difficult, finding one may be a little more difficult but try Google's shopping site at www.froogle.com
where I found two listed. I just happened to get one from a JF member who happened to have a used one on hand. New, the bladder is $150 or so.
The first photo shows the bladder itself with the old bladder still installed on the left, the replacement is in my hand. The second photo shows it installed... that nipple that leads under the bearing cap leads to it.
The part number for the air bladder, officially called an "Actuator, Air Locker" is 52104611AA.