I hate reviving old topics, but...
If you don't mind driving for a couple hours, the Pine Barrens in NJ have a ton of caches. I knew there were a few there, but today when I was playing with the Google Earth plugin for the Geocaching website, I saw over a hundred in the area of Wharton State Forest, one of the legal offroading areas. Depending on when you go, and which areas you visit, you'll need good mud tires (and, of course tow points, and another Jeep with you to pull you out) as many of the trails get very muddy.
The local offroad clubs are doing a Trail ride/Cleanup this weekend, and I'll take my brother (geocaching partner) and mother out for a little geocaching+offroading while the offroad clubs are having fun on the trails.
Note to anybody wanting to do 4x4 caches:
-make sure your GPS is tracking your route as soon as you get off the main roads (so you can find your way out)
-go with another 4x4 vehicle (so you can get pulled out).
-don't forget to bring food and extra clothing incase you get really stuck and have to stay out overnight.
And, of course, make sure the areas you'll be driving on are really approved for legal offroading. (The popular offroading area near me is only approved for driving on the established trails, a lot of people don't know that, so they get a nice ticket from the police. My brother and I spent a nice rainy day caching in that area, picked up 10 nice caches, and stayed on the legal trails. Fun!)