Here is mine with no load and the load on it. I just can't see me needing the assistance in the rear. This is also with 100 gals of water on board the trailer.
If their feeling some shutter on the 4dr long wheel base and not on a 4dr short wheel base the issue could be at the drive shaft. I am just guessing here as I haven't even gotten under my truck yet. But if the long wheel base has a 2 piece shaft with a carrier bearing it may be coming from that bearing. High torque can make them shutter in a off angle. That's just assuming....
Hard to tell with your pictures just how much it drops. Mine dropped about 3-4 inches, don't remember exactly how much. With 3000+ plus added with the trailer load it would put the rear of the truck below level and shoot the headlights high.
I have the shutter when loaded. Getting the dealer to look at it turned out to be more hassle then it was worth - since they can't reproduce it and in general made it lees then simple to bring the trailer with me for the test drive. With air and the truck leveled out it still shudders. Some on the Ford Forums have had drive shafts replaced and solved the issue. There seems to be a 50% cure rate with the addition of air bags. SRW's seem to be less prone to the shudder, but they sit 2" higher unloaded, and that may help.
If somebody can tell me what the actual weight of a 12 F350 4dr 4wd DRW is I can tell you within a 100 lbs or so of what the pin weight is.
I can tell you a 2011 DRW with a tool box and roll up cover (250 pounds before any tools), TS3 hitch (300) full 65 gallon fuel tank, some fire wood, 2 dogs, 2 people comes in at 10K on a CAT scale
For those with air bags, besides the benefit of leveling, is there an improvement in towing, via less movement?
I found with my GMC an unloaded benefit. With the bags at the minimum pressure of 2-5 psi I felt a better ride unloaded. Just that little bit of air softened it up a bit. Headlight aim is a plus and what seems as less of a "hard bounce".