Western star 4964 suspension problems

Too much weight on one side?

 

If it's leaning down in the right front look hard at the left rear. If you have a weak steer spring set there will be a slight flat spot in the arch of the springs.

 

I have a 98 4964 ex, had Springs replaced twice now. Both sets were factory specs. I have crushed both sets to the point de arching them. I never run over 12k on the axel which is rated at 12k. Look at your front Springs again and check for this condition. I am going to add a leaf to yet another set of new Springs very soon. Crushing the springs generates other problems as well. Ride comfort, pre mature shock failure and other conditions similar to what you have described. The shocks just dampen, they do not hold up the weight of the truck, the springs do.
My truck has body roll associated with weak front Springs.
What type of rear suspension does the truck have?
Have you looked at your cable mounts, torque rods etc?