With this comment it is clear that the western tribes do not think that the land east of the Jordan is truly land upon which God’s people ought to be living. It is not the Lord’s possession in quite the same way as the land to the west.1 The concept of uncleanness probably has to do with the location of the tent of meeting. Since the altar is in the west of the Jordan, that is the holy land which contains the visible representation of the Lord’s presence whereas the land to the east has no such privilege.2
19 But now, if the land of your possession is unclean, pass over into the LORD’s land where the LORD’s tabernacle stands, and take for yourselves a possession among us. Only do not rebel against the LORD or make us as rebels by building for yourselves an altar other than the altar of the LORD our God.