I have had consistent shaking/vibration issues on the front end of my truck too...very, very irritating, seeing as I how I have replaced almost everything.
August 2009: all ball joints on all the A-arms with Moog ball joints
May 2010: new wheel bearings, although did not replace needle bearings at the time...stupid
but I didn't have the parts (unprepared) and would have to been ordered
July 2010: passenger upper control arm which came with a new Moog ball joint, and idler arm, and inner passenger tie-rod end
September 2010: outer tie rod ends
July 2011: idler arm fails (due to manufacturer or my own fault for not greasing) replaced under shop's warranty
March 2012: pitman arm
What I noticed through all this? Every time a part was replaced the steering/shaking got maybe a hair better...but the main issue is still the steering box, no matter how many ways I go around it...I'm going to have to drop the ball on a remanufactured one soon seeing how I have about 1-2" of "free space" in my steering wheel movement (which is definitely out of "normal" after checking the manual!)
So since you have changed the pitman arm, but not yet the idler arm, the next bet would be the idler arm, since they usually go around the same time (that's what my mechanics have told me at least) and you may have bad bushings in either the upper or lower control arms or both. Any one of these wear items would cause your shaking or vibrations.
