TF Steering issue help

[cansled]

Having trouble figuring out the steering issue on a 56 straight axle. During the rebuild I originally kept the manual steering setup. Had the suspension looked over and everything checked out nice and tight. I had an alignment done and they added some shims to correct the wandering at highway speeds. It corrected some of the wandering but never seemed to be stable coming out of a corner.

Figuring the gearbox might be worn out causing the steering to be loose; and the fact that I was giving the truck to my 70 year old dad, I decided to add CPP Power Steering instead. The power steering install went fine, but the same symptom still exists as when it had manual steering. So now I know it was NOT the gearbox.

Here is what is happening: In town the truck turns and handles perfect. It tracks straight and doesn't seep loose or wobbly. On the highway, the truck tracks straight, but the problem comes in the corners. When turning out of a corner, the steering seems slow to react when turning the steering wheel. This causes the driver to turn the steering even more, only for it to suddenly over react and quickly turn out of the corner. This usual requires a correction in the other direction to get it tracking straight again. So every corner requires extra concentration and corrections.

It is too much for my dad is scared to drive it on the highway.....not good!
Please help!