Differential or Universal joint?

What is your question?

My truck vibrates above 60mph.I haven't had it above 70 since the vibration started.The vibration is steady and rythmic.I have been told the differential is going bad and I've been told it could be a U joint.There is no clunk when I put it in gear but there is a metallic clink as if metal were being stressed.The u joint seems tight but could it be that the u joint is dry and binding?How can I tell the difference without tearing the entire truck apart?