Have you checked the pressure in the tires? Sometimes the obvious is overlooked. I run about 33psi in my rears and they don't wear as bad now. Before I had the same thing as you, the left rear would wear really bad on the inside..... now it doesn't so I guess it was bad tire + too low pressure (28psi).

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k