How to Dolly Tow Unlockable Sterring Wheel Car

I have dolly towed 2 different cars on an older Demco and have never had a problem like this in 6 years and 60k miles. Take that chevy to a competent frame shop, tell them what its doing and let them set the toe-in on the rear axel. If that thing is not running straight and true something is wrong with the car. Get someone to drive it down a straight road and see if it "dog tracks" or runs in a straight line.