[BigAls87Z28]

I always consider NB to be "North" jersey.
The waistline of NJ stretches from Trenton to Asbury, basicly 195. 195 starts at ext 98 on the parkway. So taking Jake's idea of 57 exits per section, 127 would be the farthest north, or around exit for the turnpike, and farthest south would be exit 69, or a few miles above Manahawkin/LBI.
That would still leave a lot of people north of NB in "North Jersey"

Or you could just cut it up by groups of where most members live.
North Jersey members live around the Wayne/Paterson area, the Northeast section of NJ.

Central Jersey members are mostly Middlesex/Monmouth/Mercer county as that is the next major concentration of members.

And then there is South jersey, where everything is spread out, there is no indoor plumbing, people drive without rear brakes, roads are not paved, or you are in the military.