International lonestar hard to keep in lane

Hello to all the brother drivers i really need help and advise here ., so i got this truck for like a year now ,,in the past couple of monts i notice that the truck was doing something funny and is a little hard to explain unless you're driving to feel it '' but basically the truck feels like swinging on the rear axels '' like you're driving and you steer to the right and you can feel the back of the truck goin the opposite way '' when is loaded is not as bad but it does the same ' I've done a lot to the truck trying to fix this but i haven't got to the problem yet'' any help and advice are welcome

 

Alignment was done , 5th wheel never dry , and suspension has been checked at a couple of mecanic and also the international dealer. , but they all say not to see anything wrong but the problem is there so there might be an issued ,, i feel that it might be suspension couse when its heavy it handles a litte better.. Also been checking steering sistem but it really dont feel like a front ploblem

 

It does the same with or without the trailer

 

Yes sir i allways do alignment on all axels

 

Will have to check that but i really dont think that the problem is the cab ,it feels went steering left or right that back axels are like trying to past the front axel ' like if they are not following the steering wheels or front axel line