Hey Fast,
I had a I-cube van once that would shake and shimmy so bad when you put the brakes on, you would have to almost stop to get it to quit. It ended up being the idler arm. I would say the steering box could cause this also. Check the idler arm first.
The pitman arm, make sure it is the right one, same as old one. Then make a mark on the shaft and old arm before you remove it. Make sure the wheels are on some ramps or something to keep them from turning once the arm is off. Tie the steering wheel to keep it from moving. This way, it should go back on in the same position as the old one if nothing moves in between. A good thing to do is find the center of your steering wheel first. Then tie the wheel.
If you change the box, just center it and you should be good to go.
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Mike
1985 Chevy C-10
Mike's 73-87 Chevy Pickup Site