I would suggest a higher end shock since you/your father does that heavy towing. Take a closer look into bilsteins (I have them on my D) or some self adjusting Ranchos. I believe they are the 9000 series. A little more expensive but they will help hold up the load of that trailer. My body roll dropped dramatically when I installed my bilsteins.

P.S. I have all of you beat my stockers were shot when I bought the D at 30k miles. >:P