2004 Ford Escape 3.0 Engine Vibration

While I was driving today out of no where my check engine light started to blink and
the car developed a vibration similar to when you have a  lose tire.  It had snowed 3
inches over night here and I was driving on the interstate after the roads where
cleared.  The shaking/ vibration when away when you let off the gas or when you had
higher RPMs.  Speed didn't matter on when the vibration happened.  I thought that
maybe it would be trying to kick into 4X4, so I flipped it from AUTO to 4X4, the
problem didn't go away.  There is no snow pack in the tires and it didn't matter when
the roads where clear or when there was packed snow.  Any ideas as to why this is
happening?