Towing Issues: My gooseneck makes my truck rock forward and backward., sick to my stomach, trailer tire

Question
I have had my truck and trl to so many people, everybody tells me the same thing.....we don't know.  I have 05 F-350 Dually & 25ft 10 ton tandem gooseneck.  My truck rocks forward and backward while the trl is empty and does it a lot worse when it is loaded.  I've re wired the gooseneck plug.  I thought it  might be the Ford built-in Brake controller, so I disconnected the trailer.  It still did it when it was disconnected.  Then I thought the break-away module might be bad, so I took the rechargeable battery out and it still did it.  One guy told me that he thought the beams were flexing on the trailer causing the truck to react that way, but the trailer has 12" I-Beams. Its not safe for me to tow the trailer loaded, the way it makes the truck react.  It does it somewhat intermittently while riding down the road.  It won't do it on big bumps or at a bridge intersection, but will do it when you don't see any bumps in the road.  It shakes me back and forth so bad, that I can't eat a big meal before riding in the truck, because it makes me sick to my stomach.  Please Help!
Wise, VA
Matt

Answer
While this is not my area of expertise, I will offer a few ideas, which you might of considered..

First of all, the pin for the gooseneck in the bed might be worn, or the receiver might be allowing it to "slop around" when hooked up..

Next area I would look at is the mounting bolts for the pin plate.. If they are loose at any point and let it move a bit you will get this kind of problem..

Third is that the welds on the gooseneck might be breaking somewhere and allow it to flex a bit..

You also might check the springs and axle bolts on the trailer,  if they are loose then it might be moving a bit, and with the length of the trailer you would get an "amplified symptom"

Out of round trailer tire ??
Severely unbalanced trailer tire ??


Since it does this loaded or unloaded, you know its a trailer problem.. You do not feel it if there is no trailer..

The brake controller, well not sure that would be a cause, as it "usually" just applies the brakes when needed..

Best bet is to have a mechanic really go over the trailer, check the tires, bearings, springs, welds, and all that.  You might find the problem that way, but it might just be one of those things where everything is good but the combination makes a problem appear.

Sorry that I could not offer more, but like I said, not my area of expertise.