Albert Barnes Commentary 2 Chronicles 34:6

"And [so did he] in the cities of Manasseh and Ephraim and Simeon, even unto Naphtali, in their ruins round about." — 2 Chronicles 34:6 (ASV)

With the power of Assyria now greatly weakened, if not completely broken (629-624 B.C.), Josiah aimed not only for a religious reformation but also for a restoration of the kingdom to its ancient limits (see the 2 Kings 23:19 note).

With their mattocks ... - Or, “in their desolate places” . Another reading gives the sense, “he searched their houses all around.”