bad vibration

Get up to say 70-75 shut off the motor and put her in N. Is it completely gone? If so then can you try N with the motor running, is is better or worse?

May be a driveshaft even if joints are OK, a front trans pump or output shaft bearing, engine balancer or maybe worse thrust main or other.

Kinda sounds like a tranny issue, i would pull the pan if everything else checks out OK from inspection pont of view. Sometimes once the pan is off you will see an issue if it is real bad.