Jury's still out on 1984 Grand Marquis lean

As I've mentioned before, my 1984 Mercury Grand Marquis displays a slight, mysterious lean.

Some community members have suggested possible causes, but they can be ruled out, unfortunately. Would have made things a lot easier.

Admittedly, one sometimes tends to overlook the obvious, but there was no 50 kg toolbox in the trunk when I did my tape measure check and there isn't one now.  

I did measure the lean at the rear bumper the first time, but a re-check at the wheel arches helpfully suggested by a community member corroborates the initial disparity of 3 centimeters or 1.18 inches, respectively, between the left and the right rear side of the Grand Marquis.

I'm now considering a suggestion of how to correct the effect, but still welcome other possible answers as to the likely (or unlikely) cause of the problem.

Again, many thanks for your efforts.