So I swapped my NP-231 out for a 242 and believe the kit I got came with the wrong output shaft seal, it just puked t-case fluid everywhere. I had read about some people running them low on fluid and figured I could make a trip up the mountain, it made it up, but on the way back down I heard a thump, and then the vehicle started bucking at around 55 mph. I had it towed home and tried driving it around the neighborhood, at low speeds, I don't notice anything, but around 50 or when taking tight turns, I experience the "bucking". I took it into the shop and they found metal shavings in the transfer case fluid...

So I'm wondering, is it just stuck in 4wd (that might explain the bucking), am I missing a tooth on one of the gears or something worse? My first thought was that it was a U-joint, fuel pump, or one of the mounts, but that wouldn't explain the metal in the t-case.

 

Well sounds like you are saying it ran out of fluid, or got very low on fluid, right? Now metal debris is found in xfer case. Well if it is lots of metal bits, or chucks of metal, that is a sign the xfer case needs servicing for sure. I few fines of metal can be considered normal specially if the fluid change interval is lots of miles. But a huge amount of fines, or any large chucks of metals in the draining of the xfer case are a sign of something gone wrong and likely still going on very wrong.
Could running low or out of fluid, causing damage to xfer case cause xfer case to get stuck in 4 wd? Well I suppose that is possible, depends what seized or broke.
It should not be difficult to determine if she is in 4 wd or not if she is still drivable. You may also have some linkage issue rather than an internal to xfer case issue as for not shifting out of 4 wd.