[gtmud]

YEs if you installed the 4" lift and not a new CV axle than a vibration is likly at around the speed you are mentioning. Ball joints, just jack the axle up and take a long bar under the tire and lift up. HAve a buddy look to see whats moving. BECAREFUL!! its a bit more complicated than just R&R the ball joints. The uppers are your camber/caster adjustment. IF they are bad and the writing on the b-joint is illegible. Take it to an alignment shop, it will be a bit $$. If you can make out the writing then get the EXACT same one to replace your worn b-joint.