I'm looking for some advice from you guys because I'm not sure if the problem is with my truck or with my tire shop.

The truck is a '51 3/4 ton with 16" radials all around. I had the steering checked and the tire shop had to install a new (actually, used) steering arm. The problem is that now the truck wanders all over the road, but only at high speeds (50 mph). Should the truck go back to the shop, or could the problem be something else. At lower speeds it steers fine.

Thanks