Always wanted to hike to the other dunes at the mouth of Marble Canyon, on the west side, south of the main road. It looks to be about 6-7 mi. by the shortest route through the valley, and maybe 10-12 mi. (depending on 4wd access) by going down Marble Canyon from the Saline Road.
EDIT: I think these would be located at right background in Clydascope's excellent picture.