So for general interest, in Illinois:
I called both the DMV and the Social Security office.
In both cases, it is perfectly okay to walk in with a 13-years-old divorce decree containing a formal order to restore a birth name and restore your name. I'm told there should be no hassle, so we'll see.
To reiterate: it appears from my two phone conversations there is no statute of limitations on restoring your birth name if it's part of your divorce decree.
I'll post results once we've actually gone through with it.

