Repeaterbook, I find, doesn't accurately represent my area to it's full potential in terms of repeaters. Sometimes the HAMs in the area are also GMRS. (A guy known for his HAM repeaters in my area ALSO has a GMRS repeater not listed on RepeaterBook, only here.
Considering GMRS is very popular among hikers, groups, families, etc. I'd assume Joshua Tree would most likely have a GMRS repeater. I don't know what radio you have, but some are able to detect CTCSS tones on a repeater, and then subsequently set your radio to the correct tones. So perhaps hit up the repeater channels. See if you get anything back.