Those are kingdoms, not tribes. And as for that, we know that refugees from the northern kingdom came south and lived in Judah. Thus when Babylon took Judah into captivity, it took all of the tribes.It's important enough for Jeremiah, and Ezekiel to point out that God will reunite Israel and Judah when he makes a new covenant with them.
There is no commandment given to any non-Jew to keep the sabbath. The first recorded incidence of keeping the sabbath is when the Jews are in the wilderness and are not to get manna on the sabbath.I'm looking at the fact that the Sabbath was created long before there was a Hebrew, or Israelite or the tribe of Judah. The fact that God makes a covenant with his chosen people doesn't negate the fact that one doesn't have to be a Hebrew, Israelite, or a Jew to keep and observe it.
The Sabbath wasn't just made for Jews, but for all human beings. It is for everyone's benefit as are all of God's feast days.
So, yeah, the commandment to keep the sabbath has only been given to Israel. If you personally want to take a day off, that's probably a healthy thing for you to do, and I support you. But you are not obligated by God's law to do so, since you are not part of the People of Israel.