I have a feeling that the way in which the general public might be most helpful (other than producing a real lead) is if someone remembers seeing that SUV at any point that evening.
AM's vehicle was distinctive enough, and she was traveling during hours where there might have been sparse enough traffic that she may have stood out. Hopefully someone has already phoned LE letting them know they saw her SUV--wherever they saw it. This might help a lot with timeline.
The NAMUS report states that AM was wearing black shorts, a black top and a black sweater---with black heel type sandal shoes. And that report says she wore glasses (maybe when she drove?).
Her vehicle was a 2010 Acadia Grey SUV.
The SUV had to have stopped to pick JDR up, somewhere. So this might have been at a shop or fast food joint a long the way. And then the SUV definitely stopped on I-10 just west of the Oak Parkway turn off.
There are a number of routes to get to that point on I-10. Highway 111 can get you there from Palm Desert. You could go through the mountains, and the Pinyon Pines area by way of 74 and 79.
Maybe there is someone out there, who did see that SUV and doesn't realize that the sighting was important.
http://www.riversidesheriff.org/press/chu17-0512.asp