Went to get alignment truck pulles left replaced l/s calliper and brake line already.Of course they came out and said ball joints bad dealer replaced them 13k ago.Yea they have up and down movement but no side to side and no tire wear.They cranks it past specs on caster still pulls left.When I have 5iver on back it drives straight as an arrow.??

 

Did you try rotating tires? Might explain why it drives straight with some weight on it. It did happen to me once, I couldn't believe it would make that much difference with fairly old tires.

 

Switched steer tires allready no change.

 

A big key is that it resolves when the bed is loaded. Check the rear suspension for problems: sagging springs, wheel bearing, tire, etc.

 

What brand and size tire are you running?

 

stock size general tire.