Aragorn's reference to having visited Moria earlier is one of the remnants of the story of
Trotter — the Hobbit ranger (and at some point Bilbo himself) that Tolkien originally intended the Hobbits to meet in Bree.
Trotter had been kept captive by ‘Black Riders’ in Moria, which is also the history behind Aragorn's painful memory in Bree as they talk about the Black Riders, Aragorn having announced that ‘They are terrible!’:
Quote:
‘The hobbits looked at him, and saw with surprise that his face was drawn as if with pain, and his hands clenched the arms of his chair. The room was very quiet and still, and the light seemed to have grown dim. For a while he sat with unseeing eyes as if walking in distant memory or listening to sounds in the Night far away.
‘There!’ he cried after a moment, drawing his hand across his brow. ‘Perhaps I know more about these pursuers than you do. You fear them, but you do not fear them enough, yet. Tomorrow you will have to escape, if you can. Strider can take you by paths that arc seldom trodden. Will you have him?’
The Lord of the Rings, book I chapter 10 ‘Strider’
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The history of the writing of the first book of LotR is curious — the first chapters were re-written several times, and the Company of the Ring had reached Moria before Tolkien started anew, making Trotter a Man instead of a Hobbit. All of this jumbled history has still left traces of back-history in the published book such as Aragorn's memory of pain in Bree and his reference to entering Moria previously.
We can, of course, begin to make some guesses based on the earlier history. Trotter, the Hobbit, was caught by Sauron's emissaries. In the first version he was, IIRC, caught in or near Mordor, and there tortured, but later the place of his capture and torture changed to Moria. This was while he was tracking Gollum, and it would, perhaps, not be illogical to presume that Aragorn had also entered Moria while tracking Gollum — it would certainly not be unreasonable to search for news of Gollum in Moria, and so that might have warranted Aragorn's sojourn. However, I don't think we can simply allow Aragorn to adopt all of Trotter's story, since I am sure that Aragorn was not caught and tortured in Moria, and most certainly not by Nazgűl.