i'm looking for some places to go too. i just picked up my tj a few days ago so i haven't had a chance to get out with it anywhere yet. i found this site that has a list of some of the 4x4 trails, but unfortunately for me most of them are out in western MA. the only one somewhat close to me is Mother's Brook Path in Freetown State Park.
Massachusetts Off-Road Trails, 4X4 & ATV Trails
if you click on a trail name a list of different kinds of trails will show up on the left. if you click on 4x4 it'll show a list of the 4x4 trails in that county.