What Kind of Acacia is This?

 Q. I need to replace this acacia tree; in a very shaded area of our yard. Is it a shoestring acacia?

A. Probably not. I posted the Shoestring Acacia flowers below. It is native to Australia and commonly flowers in mid winter in Las Vegas, north of the equator. 

Shoestring Acacia flowers

Most likely it is called Mulga or Mulga Acacia. Check out Mulga aneura from this website in Phoenix/Tempe.

http://www.public.asu.edu/~camartin/plants/Plant%20html%20files/acaciaaneura.html