If the tyre has never been off the back, it has to be either a bent axle housing or a loose wheel bearing (or very unlikely bent or loose stub axle). If it got that way on the front (much more common) there are more possibilities - last time it happened to me it was worn swivels.
There is no adjustment in the rear, and worn bushes etc would be expected to wear both rear tyres.
Of course another very unlikely possibility is a faulty tyre.
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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