The most direct route to take going to Palm Desert and Joshua Tree is:
West on I-40 from ABQ to Flagstaff, AZ...
South on I-17 from Flagstaff to Phoenix...
West on I-10 from Phoenix to Joshua Tree and the Palm Desert area ...
If you don't mind CA freeway driving you can proceed west on I-10 from Palm Desert and pick up I-215 south to I-15 south to the San Diego area.
If you'd prefer not to get into so much of the CA freeway scene, you could backtrack a bit from Palm Desert to Indo and take CA 86 south between the Salton Sea and Anza-Borrega Desert State Park to El Centro and then I-8 west to San Diego.
If you want to take in Anza-Borrego Desert State Park (beautiful), instead of going all the way south to El Centro on 86, cut west on 522 or 78 and make your way to Borrego Springs...there's a nice state park campground there with hookups. From there you could go back to 86 and on south to El Centro, etc ... or cross over the mountains from Borrego Springs to the Julian, CA area and meander your way to San Diego. There is a climb out of Borrego Springs to Julian, but totally doable.
We have driven all of this except (1) the stretch from Indo on 86 to 78; (2) 522 to Borrego Springs and (3) the I-215/I-15 route south from the LA Metro area to San Diego....
TB & Greg and Abbey Schnauzer
AirForums #21900 . Membership Chair, 4CUnit #3954
Travel Log: AZBambi...On the Road Again