1991 Acura Legend Vibration

Shakes or Wobbles problem
1991 Acura Legend 6 cyl Automatic 190k miles

I have a front end vibration that my mechanic hasn't been able to diagnose. He has only been able to tell me that its not the obvious, i.E, not a ball joint, axle not alignment. It only happens on longer drives (40+ miles) and only when accelerating. It starts in the right front wheel with an out of balance "thumping" feeling that develops into a scary wobble/shake that radiates to the center then left side of the front end. Its not something I feel in my steering column until it radiates to the center of the front end. Your suggestions? I appreciate the input.