1)
Why does it say "Noda bi'Yehudah"?
1.
Radak: When they will see Hashem's vengeance against the nations that gathered against Yerushalayim [He will be known]. It mentions Yehudah, for Yerushalayim is in Shevet Yehudah, and the kingship (capitol) will be there.
2.
Malbim: Shevet Yehudah served Hashem, and He was known among them.
2)
Why does it say "b'Yisrael Gadol Shmo"?
1.
Malbim: Among Yisrael, i.e. the other Shevatim, Elokim was not known so much, but even so, His name was great among them.
