Hi Kristie, there are some very knowledgeable ppl here, but also just
fyi there is a excellent (free) forum over at
http://www.gm-trucks.com/forums/
... select the  Light Duty  forum.  They also have a good search
mechanism, I know I've seen the EVO sensor talked about a lot - I
think there was even a thread with pics about how to fix it.

If you go to the dealer, (which is about the only place that you
probably will find the part), they will know what you are talking
about, as we have sold literally thousands of them.  The EVO
sensor is a "sensor" that sits at the base of the steering column.
It measures and relays the speed at which you are turning the
steering wheel...which in turn tells the valving system in the
power steering pump how much assist to provide the
driver depending on road speed and how fast you are turning
the steering wheel.  When the sensor goes south....it will often
cause the steering to feel weird at certain speeds as it commands
full assist when you don't really need/want it.  As a result, you
will feel as though the steering all of a sudden over assists.

Replace the sensor and all will be well.

Ian