They probably are a yucca, but there are many different species in Arizona. I'm no expert on Arizona yuccas, but if they are low-growing/trunkless, and have narrow leaves they could be Yucca angustissima or close relative, but if they have wide (over 1/2 inch wide) fleshy leaves they are probably Yucca baccata. The trunked yucca in that area would be Yucca elata, but these can grow huge with multiple heads. You can do some google image searches to verify.

Thank-you for responding. As we're now in Phoenix and making our way north again, I'm not sure I'll see any more, as they seemed to be more common further south, on rocky hillsides and washes. Many had trunks with the old leaves were still attached. They had large dry stalks, often still with seed pods. Could they be Yucca elata?

Yucca elata is definitely a common semi-desert plant, so it would be more abundant above the low desert proper (Phoenix area). I think all the trunked yuccas in the US southwest generally keep their old leaves, but Y. elata is further distinguished by it's especially tall seed stalks and thin straight leaves edged by white peeling fibers. Other trunked yuccas have much thicker leaves and shorter flowering stalks.

If you were wondering, Y. elata, also called soaptree yucca, is the state flower of New Mexico, and here is found mostly in semi-desert grasslands throughout the southern two thirds of the state. It is extremely cold hardy, having naturalized as far north as Denver. It also grows well from seed, and a large old specimen with multiple flowering stalks in bloom is a fantastic sight indeed!