1994 Nissan Sentra Persistent Vibration with new tires and

My 1994 Sentra has a persistent front wheel vibration with new tires and rims installed. Old tires were half gone after 5,000 miles. Everything looks firm except some play in one of the axels. My 15 year mechanic does not want to condemn the axel since the vibration continues down hill and without power applied. The vibration is worst between 60 and 80. What could be the problem? He has examined, rebalanced carefully, aligned, changed a bad ball joint.