So I've had this 1991 240 wagon for about a year (only summer driving) and I've noticed that the rear tires are cupping, wearing through, and blowing out. The first two blew out, the next two I caught before they blew and they were badly cupped.

In the meantime I have replaced front tie-rods and ball joints and had a 4 wheel alignment done. I asked them to look for bent/broken/worn rear suspension components during the alignment and they found nothing (who knows how hard they looked).

A few hundred miles later I can already feel the cupping on the outer edge of the tires and I can feel the vibration while driving. It feels like there is a lump on one of the tires.

So what could be causing this? I assume there is more than one potential cause but the fact that it is BOTH rear tires seems to be a clue.

Thanks!
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'91 240 wagon, '89 240 wagon, '90 740 sedan, '92 740 wagon