If you can change the vibration with more or less power that is usually drivetrain trouble such as failing U-joints. You description sounds same as mine except mine smoothed out at 70. Turned out both U-joints were toasted bad and parts were falling out of them. Funny thing is that they felt tight when I grabbed the prop shaft right up to the time I drove it onto the service lift to change them.
You can try letting on and off the gas while cruising to see if the vibration changes with power changes.
If the vibration is strictly speed related I might look at the wheel balancing first.
1995 4Runner Limited V6 3.4 Liter 5 Speed 4x4
1995 4Runner V6 Auto 2WD