High Speed vibration, can't find what's causing it.

One thought I had is the rear leaves could be starting to wear out and allowing too much axle windup (dynamic pinion angle) when on the throttle since OP mentioned it goes away when letting off
calvert split monoleafs. 1 year old. I would think axle windup would be worse with more torque in 1st and 2nd gear. I think it is mph related.