First thing I would check is air pressure. I know that if I don't run the rear tires at 34 or 35 psi on my 300D they wear on the inside, especially the left rear.

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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