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Yes that can be scary to drive that way. Take it to a mechanic and have it diagnosed and repaired.

Here is an informational post from:

The steering-position sensor is used to tell the computer the position of the steering wheel and how fast the driver is turning the wheel. Based on that information and also on vehicle speed, it will vary the steering effort by controlling the bypass orifice to relieve powersteering-pump pressure. There are two likely conditions that will cause the system to malfunction - a lifted suspension or oversized tires with extreme rim offsets. Both of these will change the force to the steering gear and cause a hydraulic oscillation. In layman's terms, the steering wheel will shake.

The easiest way to diagnose the problem is to apply the brakes on dry pavement and turn the wheel lock to lock (while the vehicle is stationary). If it takes some strong-arm work to move the steering wheel, then the pump pressure is not high enough. Correct that first and then look at some type of tuned hose that acts as an accumulator to dampen the hydraulic oscillation. A spiral-wound spring inside the pressure hose will help dampen the fluctuation of pressure. You should be able to find a local custom-hose builder to do it. You could also temporarily install stock-type and stock-size tires up front and drive the truck to see if the problem goes away, which would further help you isolate the source of your shaking. - K.F."

You might, just as an experiment as suggested in the above, disconnect the electrical connector at the actuator (rear of the pump) to see if that eliminates the problem . If it does, you won't know if it's the actuator or the EVO sensor, but at least it puts you in the ballpark. Let us know how it turns out.
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