From the DCNR website:
ATV Riding
"Only Class I ATVs (motorized off-highway vehicle, which travels on three or more inflatable tires and has a maximum width of 50 inches and a maximum dry weight of 1200 pounds) may be operated on state forest trails designated for ATV use.
Class II ATVs (motorized off-highway vehicle, which travels on three or more inflatable tires and has a width which exceeds 50 inches or a dry weight which exceeds 1200 pounds) may be operated on state forest land ONLY if the trail is specifically posted open to Class II ATVs of a particular width and weight."
So it looks like only certain trails are open.