Why does it say "Tik'u Chaf"?
Rashi: Join together [to joyously shout to Hashem in song].
Radak: Yisrael will tell all the nations to shake hands and rejoice with us.
Malbim: Nowadays, the nations are constantly at war with each other, for two reasons. (a) Emunah - each creed fights and wants to force those of another creed to [accept] their beliefs. (b) Every nation wants to rule over other nations. In the future, "Lo Yisa Goy El Goy Cherev" (Yeshayah 2:4); rather, all will shake hands to make permanent Shalom between them, to cease sword and war. Then all will serve Kel Echad, and Hashem will be King over all of them.
Why does it say "Har]'u"?
Malbim: The norm was to blow the Shofar when appointing a new king. Then, all will declare Hashem to be King over the entire land. It will be a great Teru'ah; they will no longer be different Emunos and camps.
