I know some folks catch crappie in Black Creek north of Green Cove Springs. Another decent option is Julington Creek near the Fruit Cove/Mandarin area south of Jacksonville. Head upriver at least to where Julington and Durbin Creeks split. You are correct in that stretch of the river is somewhat brackish, and you will likely find saltwater species in addition to the usual freshwater suspects. The tide has influence there too. If I were targeting crappie, I'd probably focus on stretches of river where you find typical freshwater plants (pads, etc).