It is most likely to be the thrust washers in the centre diff that have chewed themselves to pieces, particularly if you like to keep a front wheel spinning while going up steep slopes etc.
You can replace the thrust washers without removing the whole transfer case, although it also may have wear in the other thrust washers in the case.
To do that you must remove the front nose of the transfer case which is a bit of work but not as heavy as removing the whole case.
Regards Philip A

