You have to find a dock or a bridge that is off salt water to drop traps. Typically a creek of river.
As the water gets colder the crabs become lethargic and don't move much. To the point the trap pracatically has to fall next to them. Water's in the 50's its going be a chore to get more than one or two without looking for new spots.
Ideally July thru September is good. And it must be salt water. Too far up the creek into a swamp and the salinity goes WAY down.
Sorry I can't give specific spots in south NJ, b/c I'm not from down there. But you may want to scout some place out in the off season. Low bridges, public docks on creeks, rivers or on bays.
Good luck! Perhaps bum a ride with someone on this board?

Capt. Frank
"Heat Seeker"