There are supposedly some nice trails up near Mahwah. I've only seen them online, Stag Hill is the area. Punch up Stag Hill on You Tube.
For something a little further of a ride check out Wharton. down in the Pinelands. Really nice.
Check out some NJ Jeep forums. There's always some people going out wheeling. Don't go by yourself, you could get stuck.