Front-End Vibration

We love the ride of our '99 Millenia. The smoothest of our three vehicles.

Recently when driving on Interstate, I noticed front-end vibration between 60 and 70 mph.

Nothing below 60 and smooths out at 74+. When I accelerate from 64 to 68 vibration is extreme.

Had two older tires replaced, rotated, balanced, and four wheel alignment. No improvement. Any suggestions ?