I fixed it. With the shackle off, the orange rubber bit where the shackle foot goes in slid right off. This exposed an open end. With the rubber piece off, I could reinsert the shackle and snap the retaining ring back into the groove from the open end. The only problem I ran into is that I couldn't just slide the rubber piece back on. I tried to snake it on over the length of the shackle, and it ended up snapping at the thinnest part. It still stays where it's supposed to, it seems. Hopefully it will continue to work. I might take it apart and post pics if anyone's interested.