Is chickenpox still prevalent in the us?

Yes. Mostly, new cases have decreased: that's incidence , and is due to the vaccine. http://well.blogs.nytimes.com/2012/08/20/chickenpox-down-80-percent-since-2000/.
Declined but present. The incidance of chickenpox has declined about 85% from introduction in 1995 to 2004. That translates nationwide to a drop from 2 million down to 300, 000. See aap/redbookvaricella-zoster.