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5.0L (1979-1995) MustangTechnical discussions on 5.0 Liter Mustangs within. This does not include the 5.0 from the 2011 Mustang GT. That information is in the 2005-1011 section.
I will say that I am 80% sure it is your balancer. If it is the factory balancer I would change it anyway. I have seen many balancers that pass a visual inspection but the outer ring spun perfectly on the hub.