The darker, rust coloured part is called a slip yoke.
As your rear wheels move up and down due to suspension movement the propellor shaft moves up and down and this slip yoke goes in and out of the transfer case because it is attached to the shaft.
The slip yoke has a seal in there to keep the fluid from coming out of your transfer case.
On your transfer case you will see two largish bolt heads (or possibly two things that look like bolts with the heads cut off and a square hole inside them). I forget the exact size of the bolt heads - something like 1-1/8" IIRC. I'm thinking that your Jeep being a Chrysler will have the square hole style plug. Stick a 3/8" drive ratchet in there and it should back out. You may want to put a 3/8" extension on your ratchet to get better leverage.
The top one is to check level and add fluid.
The bottom one is to drain fluid. Make sure you have the right one before checking or you'll have ~1.5 liters of ATF fluid in your lap.
Big mess.
To check fluid remove the top plug and see if your fluid level is level with the hole. This needs to be done on level ground and you stick your finger in to feel around.
When I seal it back up I'll put a dab of PTFE paste on the threads before screwing it back in.
If you are doing this out in your laneway, throw a largish, flattened cardboard box under the work area to catch any possible oil drips.
HTH
