1999 Chevy Silverado Vibration at 60 mph

I have a vibration (felt through the steering wheel) that starts a little at 40 mph but is at its worst at 60 mph. I have had the tires checked (they have 12,000 miles on them) and even had the runout (force variation balance) checked and everything with the tires and wheels seems to check out OK. Could this be a suspension issue? Maybe a ball joint? My tire place that checked the balance and runout on the tires and wheels indicated that the upper ball joint on the passenger side was worn and looked as though it needed replacing. Could this be causing the problem? Thanks guys!