We would like to take a 1/2 day, sunset, or sunrise tour of Monument Valley when we will be there in May. Can anyone recommend a particular tour company?

Simpson's Trailhandlers -- hands down! Ask for Richard.

I can recommend these three companies, based on many trips to Monument Valley over the years. All are run by Navajos who grew up in the Valley ...

Monument Valley Tours, which used to be called Totem Pole Tours ... I've known Vergil Bedoni for almost 30 years and he's a great guy and excellent photographer. He handles logistics for movie companies and does a lot of business with European customers too. I think his great-grandfather was the local chief in the 1880's ...
http://www.moab-utah.com/totempole/index.html

Daniel's Guided Tours ... Daniel Chee was a guide working for Vergil before going off on his own about 20 years back. He charges less than some of the other guides so worth checking out ... he has worked with a lot of pro photographers and also appeared as an extra in such movies at "Back to the Future III" and "Forrest Gump" (but it hasn't gone to his head) Another really nice guy.
http://www.jeffmantell.com/Site_2/Ab...__Contact.html

Fred's Adventure Tours, operated by Fred Cly. There's a Cly butte you will see on a tour of the Valley, which was named for one of his ancestors. Fred also works with pro photographers, has appeared as a stuntman in a couple of movies, and twice I saw him in TV commercials, so he also gets around.
http://southeastutah.org/en/entities/172/71/

Thank you both for your suggestions. I will check them out.

I agree with Simpson's. We've toured with them twice, both times with Richard. The first time he sang a Navajo song, the second time he played a flute. Excellent tours both times.