Well, used transfer case would be the last option to consider for 2 main reasons: Its bulky and heavy= expensive and sloooow shipping and you will never know if this bearing inside is ok. I did some research and it turns out that there are 2 main reasons for ATC400 transfer case failure here. An electric motor mulfunction that leads to permanently engaged 4wd and death of the friction pack. second - the shown bearing falls apart. We have an idea that it could happen due to the very cold temperatures (this winter was extremely cold often lower than -35 -40 C) At these temperatures at the beginning of the ride the case is not properly lubricated due to the construction of the transfer case.
My total mialage is 50 000 miles. Bought in in CA 2,5 years ago with 35k miles. Black/terracota 3,0 l. I tried to change oil before taking apart transfer case but noise did not go away. I did use a specific BMW oil and also did programming that resets TC.
Old oil was just dark, nothing wrong, level full