I had one tire do that once, right rear, and it was a toe issue. Basically it was dragging the tire sideways a bit per rotation because the tire was pointing a bit to the right, so a "toe out" condition. I would've assumed that would cause it to dog-track and screw up the inner edge of both rears, but the left was fine. Maybe it's a matter of which tire has more traction wins, and the other drags and wears. I can say it cornered much better after it was aligned, and of course the wear issue was gone. Unfortunately I tossed the alignment data sheet out a couple months ago, but I think it was 1/4" off? So basically dragging 1/4" per rotation which adds up quick. Mine was a '13 E350, which I think is a C chassis? I dunno, but either way I imagine it isn't much different than yours.
So my thought is you hit something, curb, pothole? I remember my mom knocked her left rear alignment out bad and insisted she hit nothing so it must've been my fault. She's hit many things and totaled several cars, but I drove her car one time and it was my fault.
The only cars I'm aware of that can't have a rear alignment are straight axle, even thou you can anyway because I've done it. Even IRS with no normal toe adjustment can use concentric bolts. Or worst case, offset bushings. The alignment shop will know what to do so no worries. If you do it yourself then you've got some work ahead of you, but toe adjustment is fairly easy.
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