J stands for 井, which implies 发射井, or “launch silos”. It means a DF-31B (东风31乙)designed specifically for silos. It confirms previous analyses pointing to those ICBMs loaded into the wind farms being DF-31s. Since it is a lot bigger than the previous DF-31A, I would assume that the range of the DF-31BJs would be greater than the 11,700km range proven by the DF-31AG.BJ?
Already these missiles look a certain way. PLA namers have a sense of humor.