Are there any pre-print services that accept manuscripts already accepted by a journal? My publication was accepted by a journal, however I want to post it online prior to publication.

BioRxiv was the original repository of choice, however they do not accept articles accepted by other journals.

Question:  Are there archives that accept pre-prints that've already been accepted by a journal?
arXiv has no such restrictions as far as I know.  If your copyright agreement with the journal allows you to post the article on preprint servers, then arXiv will happily accept it before or after acceptance / publication.  

When posting to arXiv, you'll need to choose a license that's compatible with your agreement with the journal.