Could be a number of problems, naming a few: Spring pack center bolt moved/sheared on that side. Axle spindle assembly bent causing incorrect camber and toe in/ toe out, problems with shackle bolts or hangers on that particular wheel, excessive slack in wheel bearings, last but not least, defective tire and/or wheel.
An alignment check would be the first step to take to ensure those axles are in their the correct positions. Hope you find and can correct that problem with ease.
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