I probably should not be the one to encourage you to go alone, since we always tell people not to do that, but . . . I've done both of those routes alone (well, had my wife along for company, but just one truck). It really all comes down to how far are you willing to walk if you get stuck (or break down) and can't get help.
Fish Creek is pretty heavily travelled, especially on weekends. There are a few sandy spots, so be careful in 2WD. Might help to have a shovel and your own tow strap in case you need to ask someone for a pull. We had a 2WD X with our group 11/25 that had no problems at all
Palomar Divide Road is paved part of the way. I did it years ago, back before any of it was paved, in an old Datsun 2WD truck, up to the Barker Valley trailhead without any problems. The shelf road section does get pretty narrow in spots.
There are some other scenic backroads around San Diego that are fine in 2WD, you might want to look for Tony Huegel's book California Coastal Byways, or his other book, California Desert Byways. I'll let someone who knows how link in a photo of the cover. Check out Boulder Creek Road, from Descanso up towards Julian, before it starts snowing. Also, some interesting roads around Black Mountain (the one near Ramona, not the one just off I-15 near Poway).
Keep checking back, as I'll be planning another few Anza Borrego day trips soon. Thinking of Blair Valley and the pictograph hiking trail. I hear the driving portion is 2WD friendly. Also want to go back to Sandstone Canyon again this season.
If you can make it to the Xmas party we can talk more about what's in San Diego then.