I used to have a front end shop and still do rods and custom suspension mods.
No doubt there is a box problem.
Alignment gets a lot of blame for the rear end wandering. You'd be suprised how much a rear end with rubber bushing flexes and steers a car.
I suggest installing urethane bushings in the rear to prevent wandering. r
You'd also be suprised how much a panhard bar helps a leaf spring car.