How are the pieces glued? Are they glued such that the glue lines run parallel to the floor? Or are they glued and stacked vertically?
New glue doesn't adhere well to old glue, so Steve is correct in suggesting splitting them all apart, cleaning off the old glue by scraping or planing, and regluing them. This will take some effort to get it all off and down to the original wood surface. You can use an epoxy which will all but eliminate the old to new glue issue, but you will want clean and even surfaces.
I would like to see a photo of exactly where the splits are and to what extent before giving more advice. A repair may work rather than a complete tear down... I donno? If the splits are just visual rather than structural, a repair may work OK.
Making new rockers may be easier than a teardown and reglue iof the the existing ones, but that depends on how bad the splits are in my opinion, and whether you can clamp them securely.
Another issue is the age of the chair. The original glue may be hide glue which can be softened with hot water or steam and it's easier to repair.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)