Definitely hire a guide at Monument Valley. When you get to the Visitor Center at the Tribal Park, as it's called, you can make arrangements there. They are basically independent outfits, for the most part. You can also get tours from Gouldings Lodge, but they go with a whole bus full of people. The private guys will most likely only have a few, although the vehicle might not be as fancy as the ones Goulding's has.

As in our case, we lucked out, and he had no other clients except us, and no clients booked after us, so he gave us more time than we even paid for. In fact, I was so reluctant to leave, we missed our dinner reservation at the main Lodge at the North Rim of the Canyon that evening. It was worth it, IMO.

I just cannot describe how peaceful it was there- all you could hear was the wind. Oh, and you will have red dust in your clothes and hair for ages!
I just know you are going to love the rest of your trip!
I don't mean to hijack your thread, but I just wanted to share a few more pics (I hope you don't mind

).
I could talk all day about this stuff!
Here's a cool one at the South Rim as a storm blew in:
Another one of The Ear of the Wind, as it's called; the boy is my son- he was about 12 at the time (gives you an idea of the scale):
I think Arches National Park and Canyon de Chelly have similar scenery to Monument Valley- I've never been to either of those. I have been to Zion, though- it's gorgeous! This is at Zion:
I don't know if you have been or are planning to go farther North, but I absolutely LOVED Grand Teton National Park in Wyoming.

We did a raft trip on the Snake River that was amazing- I highly recommend it!
Here's the Tetons- the Snake River is in the foreground:
These flowers are called Silver Lupines; they're not only beautiful, they smell amazing!