Update: Interesting little page on NJ:
http://www.slavenorth.com/newjersey.htm
So, they passed an 1804 law for gradual abolition (which is good, and should’ve been done nationally), or a sunset more or less.
Generally seemed to work by 1840. It didn’t take total effect, though, until after 1860, with PC terms for slavery.
And how could that have been done nationally without a constitutional amendment?