Track bar improves things in two ways. First and most obvious, it reduces or eliminates the side to side movement of the rear axle on a typical leaf spring suspension. Not so obvious is that it raises the roll center of the rear suspension from the surface of the road to the point where the track bar crosses the center line of the axle. Raising the roll center increases the roll stiffness of the rear suspension (just like an anti-roll bar does) and it makes the vehicle roll around a higher point rather than tipping above the road surface, so roll both is and seems less. You know how in NASCAR they are always "raising the track bar" in pit stops? They are changing the rear roll stiffness to make the car either push more or be looser in the corners.
Brian