Two NPS singing rangers I remember particularly well are Linda Russell and Rita Cantu. Linda was at Federal Hall in Manhattan, singing in the great accoustics of the rotunda, and on the side singing with a couple of other NPS folks. Rich Patterson was one. They had a group called Prairie Smoke, after a favorite flower of Linda's from her childhood in Wisconsin. Probably more information than you need. I know that Linda hasn't worked for the NPS for many years now.
Rita was at Great Smokies. A quick search on Rita looks like she's with the US Forest Service now.