Chevrolet Repair: 91 silverado steering, lower ball joints, outer tie rods

Question
91 silverado, the steering doesn't come back like it should when you are driving. you'll be driving down the road and it will drift slightly, then you have to compensate with the steering wheel. its tight though, no slop in the wheel. truck has been allighned, new idler arm, inner and outer tie rods, sleeves, upper and lower ball joints. still has the problem. seem like yopur always turing the wheel to compensate for it, but wheel is always real tight. it doesn't drift that much, just slightly. what else can i look for that would cause the problem? there is an adjustment on the gear box to make the steering tighter, which i did adjust. i can't remember if the drift problem was before or after the adjustment. all parts were replaced within the last week. thanks for any input. also new front tires!

Answer
Thank for being elaborate.

 After replacing all of those parts we can skip the 'jack it up and see if you can move the wheels freely back and forth' routine.  Sounds like you have pretty much covered everything.
 Steering wheel return problems can be caused by siezed front end components and one thing I just want you to make sure is that if you used second line idler arms (the two arms that make the parallelogramsof the steering system at the frame) that they are not too tight.  Did you find that it wasn't as bad before the idler arms?  I have been having a hell of a time with them lately after doing alignments.  I could tell right away when I was doing sweeps and could barely turn the wheels when they were on the slip plates.
  Anyways.  If those idlers are good you can give it a little extra caster when doing an alignment.  Caster angle is the angular displacement from the vertical axis of the suspension of a steered wheel in a  vehicle, measured in the longitudinal direction. It is the angle between the pivot line (in a car - an imaginary line that runs through the center of the upper ball joint to the center of the lower ball joint) and vertical.
 Not to beat up other shops but sometimes guys 'go for the green'.  That is to say make all of the red numbers on the alignment machine green.  (green is spec).   Just because all of your alignment angles are in spec doesn't mean that your vehicle will behave the way you want or expect.  Thats where knowledge of the alignment really comes in.  

So

- Make sure there isn't anything in the front end causing it to have memory steer
- Give it a little more caster and don't forget the .5degree lead to the left for road grade!

Wayne