First stop, check out the Northeast section of the forum:
North East Region - Toyota FJ Cruiser Forum
There are get-togethers and trail runs listed throughout the year in this section.
Rouch Creek Off-Road Park near Tremont is a great choice for a stock FJ. It's about 2 hours northwest of Philly.
George Washington National Forest is 5 hours south on Interstat 81. Very pretty and many clubs on here stage runs/campouts there.
Look up Baltimore 4 Wheelers. They are a very active club and well-organized.
Have fun!!
M