Well, as you can imagine, it's VERY difficult to diagnose noises over the Internet. Here's a couple of things. The biggest reason for NP231/242 failure is lack of oil. It doesn't hold a lot, so if there's a seep it can get dry. A good time to remind everyone to check the fluid somewhat regularly. I finally put blue B&M synthetic tranny fluid in my t-case so I could tell if any leaking fluid was coming from the trans (red) or the t-case (blue).
A typical situation on these t-cases is a worn out chain, which can be accelerated from lack of lubrication. A badly stretched chain can lay against the bottom of the t-case and make a scraping (very hard to descibe exactly) sound. I saw one so bad when I managed the Jeep dealership that the chain wore through the bottom of the t-case. If you hear a steady, gravely sound, my first guess would be the chain. You could confirm a stretched chain by putting it in 4low and getting some strain on it, like a real tight turn on a hard surface, or trying to climb a big rock, and if the chain is stretched it should pop very loudly as the chain jumps the sprocket.
Another thing is bearings, but a steady sound is unlikely. Through long term wear, or a lack of oil, the plastic pads on the mode fork and the shift fork can wear completely away. If this happens, the fork can ride against the metal of the sliding collar and make noise, even wear away the slightly softer shift fork, rather than ride on the plastic pads.
Unfortunately, it looks like you're going to need to pull your t-case apart and check it out. Might as well replace the chain and all the plastic pads, and inspect all the bearings. If there's a problem with the bearings, the input and rear output shaft are much more likely than the front output shaft bearings, since the front output shaft bearings sit in the oil, while the others depend on oil getting pumped to them.
Best of luck,
