To find a file in the archive use:
tar -xvjf backup.tar.bz2 | grep {file_name}
To restore a file from the archive use:
$ tar -xvf {tarball.tar} {path/to/file}
$ tar -zxvf tarball.tar.gz etc/some/file
$ tar -xvjf tarball.tar.bz2 etc/some/file <- did not work, (extracted whole archive)