At the time of Israel’s exodus from Egypt and their entrance into the promised land, Israel was one holy kingdom (Exodus 19:5–6).
The mention of Judah and Israel as separate lands shows the psalm was written after the kingdom was divided (1 Kings 12:16–17; compare 1 Samuel 11:8; 1 Kings 1:35), and probably after the exile. Judah became God's sanctuary because the people worshiped God there. His tabernacle, and later his temple, were located there.