I didn't read all 4 page of this thread. But, you said the only thing you haven't done is check the trailer axels for squareness and that may very well be the problem. I had a single axel trailer once with a bent axel and that caused the trailer to pull terrible. Once a new axel was installed it pulled like a dream.
Before spending any more money I would check the trailer axels.
The poster that said to try towing the trailer with a different pickup might be on to something too. Then you would know if the trailer is all of the problem or just part of it.
