300TE with steering problem

[Lovemywagon]

I have a 1989 300TE with a very annoying problem. At highway speeds on a flat road, the driver has to constantly correct the steering. It's as if the rear of the wagon is trying to steer the car. As the owner of a small trucking company, it feels like I am pulling a trailer with my car, with side-winds hitting it as I move down the road and I have to constantly correct the steering. This occurs while I am holding the steering wheel perfectly steady. I've replaced the front idler arm, tie rods, dampener, new shocks, and new tires but the problem persists. Could the rear suspension be off?

Thanks,