could possibly be your rotors. I had a similar problem and you wouldn't feel it (the shaky wheel would stop) when you would brake to stop, but it could when you brake checked or tapped them.
thought it was something with the power steering, but time came to replace rotors and i replaced the fronts first. I started to late and had to work that night so i just went with the fronts for then. the problem was still present. so i really thought it was the power steering.
the next day i replaced the rears and it was back to driving normal, somehow a rear warped rotor caused the steering to shake at highway speeds.