The 285/75-16 will definitely fit in the spare location. Mine fits, no problem.
As for trails, to start out I would say go out to Ocotillo Wells and just explore the area. Lots of different terrain and a good place to learn your rig.
I been using this book for trails in S. CA and its great:
Amazon.com: Guide to Southern California Backroads & 4-Wheel Drive Trails (9780966497649): Charles A. Wells: Books
As for trails, to start out I would say go out to Ocotillo Wells and just explore the area. Lots of different terrain and a good place to learn your rig.
I been using this book for trails in S. CA and its great:
Amazon.com: Guide to Southern California Backroads & 4-Wheel Drive Trails (9780966497649): Charles A. Wells: Books