fld120 steering problem ! pls someone help me !

Since you have confirmed that you have no mechanical binding or interference problems and the truck steers when not rolling, I would locate the best alignment tech you can find and trust to set your caster to optimum spec and I think you'll have your problem solved.
It is common for some shops to overlook caster due to adjustments being labor intensive, so they let it slide, but when you carry a lot of weight on the steering axle it will definitely show when you don't have enough and I am assuming you have a headrack which requires your alignment to be to optimum spec.

 

The steering box and the idler arm have a neutral spot, if in replacing all that you have done that setting was lost, it will make it steer goofy.

 

There was a great alignment thread in here last year. Went into more details about caster. I know some techs will set each side slightly different sometimes.

The steering box shafts have marks so you can check if the box is really centered and not just the steering wheel.

 

Talk to me!

 

Has the truck ever been wrecked and repaired. I worked on w900 tri-axle doing same thing had front frame splice drilled wrong putting all in binde had them take back to dealer that did repair, they re did work now fine.

 

If you come to Nashville, TN I can hook ya up!