Once you go beyond a certain point on the trail (marked by trees with horizontal red stripes) it becomes illegal. It isn't posted, but if you get caught it's an automatic $500 ticket, no questions asked. I just recently went there (to help pull a friend's f250 out). That place is crazy...or at least the parts I went on. Almost flipped my Tacoma.
On the "road" that isn't deemed Lower Sweetwater there's supposed to be a gate marking the legal boundary
I recommend NEVER going alone on this one. On some parts, a spotter is necessary. On other parts - depending on your chosen line - a rescue vehicle is necessary.