What I have is not information that you don't already know, just my research...which when posted into the archives might result in someone seeing it. And my suggestions.
Not finding Barbara's grave, I thought maybe she was with her parents as your family tree online shows she never married (or at least not identified in your family tree).
Her father Henry W. Grunewald is buried in Riggs Cemetery, Rosalia. You have his gravestone, but no cemetery identification. Her mother, Emma, remarried to Jacob Olson, had some children, died in Rosalia in 1923. You have a photo of Emma's stone (somewhere in Rosalia).
I don't know if you took the photo's or "hired them out". There is a Bill Paulson in Whitman County on the Random Acts of genealogical kindness that takes photo's. The site
http://www.findagrave.com/cgi-bin/fg.cgi?page=cr&CRid=77...; has some of the Riggs stones photo's, as you know Wes Hart took the photo of Henry's. If you live in Rosalia, try the local library for newspaper microfilms. Or maybe Wes Hart can do some local research, also try the County Auditor for deeds or other recorded documents.
I found a private family tree containing Barbara's name. I sent a message to the person owning the tree, boggsferrini1, to read this thread. A search identified that person had only posted twice on Rootsweb, last one in 2002. The contact request goes through the Ancestry.com system, the person must log in to see the messages. I don't give this to much possibility.
Basically, since her parents lived and died in Rosalia, she "might" be there also. What do you know about her sister Ruth?
Do you have obits for her half siblings, the Olson's? A living descendant might know where she was buried having attended the funeral.
Barbara's obit is your best shot.
Ron Bestrom
Tacoma, WA