Wow, there aren't many sandy beaches on Lake Superior. It sure looks great. I'll be along most of the Lake Superior coastline from August 9-16, starting from the Union Bay campground in the Porcupine Mountains and ending up in Paradise (including the Keweenaw).
With the extra hint of 13 miles of backroads to get there I could "guess" somewhere NW of Big Bay. I've driven all over in there (the area between Marquette and L'Anse, North of 41) but never made it out to Lake Superior at the "hump", so I don't know if it is sandy up there or not.
I do remember sandy beaches from Grand Marais all the way to Crisp Point and have driven most of the coast there on 2-tracks and ORV trails but nothing was 13 miles of backroads (of course the freeways there are dirt roads).
One thing is for sure...the U.P. is as good as it gets! Looks like you had a great time camping up there with your 2 other XJ owning friends. Thanks for sharing the pictures.