It could be front tires out of balance.
It's not that unusual for many vehicles to have a critical speed in that 70 MPH range where any lack of balance will magnify. It's caused by the natural frequency of the suspension (relative to vehicle) matching the rotating speed of the front tires, which is the forcing frequency. When the two match you get resonance, which is a fancy word for greatest vibration you can feel through steering wheel. If really bad the entire vehicle may vibrate as a wheel skips up and down off pavement.
In my van the worst speed is around 73 or 74 MPH, so I try to drive above or below that speed.
You can test if it's this by driving to speed where vibration is worse, and if you feel safe doing it, accelerate another 5 MPH or so to see if it starts to go away. Then on way back past the critical speed you should feel it get stronger again.
If your vibration doesn't diminish with even higher speed, then it's likely something else.
P.S. -- Shocks limit this effect, so may want to check to make sure they look good and are tight.