This one is used outside of the Book of Mormon, but only in materials that should have been part of the plates of brass. It's use is also a bit more diverse.
The first uses are in 2 Sam 12:31 & 1 Chron 20:3. In these cases it is used to describe the implement. David is described as using harrows and other tools to put the people of Rabbah and the other cities of the children of Ammon to death.
In Job 39:10, the term is used strictly in the agricultural sense.
In 2 Ne 9:47 Jacob uses the term in a spiritual context as part of a triplet:
harrow up your souls
be plain unto you
The term is used to describe Zeezrom by Mormon (who may have been quoting Alma the Younger, see my discussion of tribulation) in Alma 14:6 and Alma 15:3. The first of these also looks like a triplet to me:
Zeezrom was astonished
his soul was harrowed up
he was encircled by the pains of hell
Ammon used the term in his missionary discourse in Alma 26:6. He describes the converts as being safe from being harrowed up by the whirlwind because they are safely gathered. This seems to be a different, more physical usage than the previous Book of Mormon uses of 'harrow'.
Alma the Younger also uses the term in his missionary sermone. In Alma 29:9, he is concerned that he is harrowed up in his desire -- like Ammon's use this seems to be more of an active, physical use rather than the spiritual meaning used by Jacob and about Zeezrom.
Alma uses the word four times in his second telling of his conversion story; Alma 36:12,17,19. The usage in verse 12 looks like a triplet again:
I was racked with torment
my soul was harrowed up
[I] was racked with all my sins
Alma also uses the word in teaching his son, Corianton. In Alma 39:7, he uses the term in a Jacobean sense.
Mormon makes the last use of the term that I could find in Mormon 5:8. In this case he is apologizing for bringing the awful vision of carnage before us.
I was especially interested at seeing how often Alma the Younger and Ammon used the terms 'harrow' and 'rack' together.