If it was a C with an extension I could totally see that but since its a straight rail purpose built chassis I don't see it as being a big deal.I think that a WD hitch may put a load on the frame extension that it is not designed for.
Actually just found the ford chassis manual for mine and it even mentions it. Wonder if that's some sort of CYA from the upfitter?
Hitches You must distribute the load in your trailer so that 10 – 15% of the total weight of the trailer is on the tongue. Load equalizing hitch When hooking up a trailer using a load equalizing hitch, always use the following procedure: 1. Park the unloaded vehicle on a level surface. With the ignition on and all doors closed, allow the vehicle to stand for several minutes so that it can level. 2. Measure the height of a reference point on the front and rear bumpers at the center of the vehicle. 3. Attach the trailer to the vehicle and adjust the hitch equalizers so that the front bumper height is within 0–13 mm (1⁄2 inch) of the reference point. After proper adjustment, the rear bumper should be no higher than in Step 2. Adjusting an equalizing hitch so the rear bumper of the vehicle is lower or higher than it was unloaded will defeat the function