Google Search for "backscratcher" comes up with a mess of long-handled forks. Forget them.
First choice is "Someone who loves you".
Tied for second are loofahs and shower squeegees. The loofah is the dried out frame of a sponge gourd. The shower squeegee should be pushed not pulled. Experience massive relief.
When all else fails and
you're far from home,
there is the door post.
Nerve ends are spaced far apart on the back. A pointy thing isn't much used compared to a broad
scratching tool that gets it all.