Great photo. But I find it hard to say for sure whether it's a Nuttall's or a Ladderbacked. Although the buffish forehead and rather broad white stripes on the head could point towards Ladderbacked, the tail has very very few bars which fits Nuttall's.
I have never been to Big Morongo, but I have seen Nuttall's in Riverside botanical garden, which isn't that far west as far as I know. On the other hand, when I look up Big Morongo on Google Earth or Google Maps, it looks as if it is more or less where the distribution areas of the two species overlap (according to the maps in Sibley's). Perhaps it's a hybrid?