rented SUVs from Alamo in Colorado, New Mexico and California.

when I called the national reservation number I was told that they would not guarantee a 4WD, that I could put a request in, but that it would be up to availability whether I would get one or not.

My best guess is that if you rent in a location with mountains, you will most likely get a 4WD. Similarly, some SUVs tend to all come in 4 while others mostly in 2WD. Remember trying to rent in Albuquerque; the Envoys and Equinoxes were all 2WD (intermediates), while the Trailblazers came in 4WD...

I would call the local rental office and ask them.