Aragorn in Moria?

[Knight of Gondor]

Like many here, I reread the trilogy somewhat frequently. I'm rereading Fellowship now, as it has been longer than usual since I've gone back through the books. And like most things you reread, you find/learn something new, or are struck by something you didn't notice before.

In this case, I just finished The Ring Goes South, where the company is defeated by Caradhras. Gandalf is suggesting (even advising; contrary to the film, but let's not go there) taking the road through Moria.

Aragorn's gift of foresight tells him that this could rob the Fellowship of their wisest counselor, and states that he has been in Moria once before.

In my readings of the Histories and so forth, I haven't come across any explanation for why Aragorn would have entered Moria. Is there any other work which would explain this?

(Man, it's been a while since I've posted a new thread!)