I really don't know what I could have done differently short of using a different trailer. I get shivers when I think about how dangerous that was. I nearly lost control at least 10 times, I could usually power out of a bad wag but every now and again it'd get way too bad and start sending me all over the place.
Dude! Now I just may have a direction for its future ;-)
What show?
Unfortunately those U-haul POS trailers only have surge brakes. If you had electric brakes, you would have been able to tap the brakes on trailer only to bring it under control. The trailer's short length created much of your problem. Because of the weight of the old girl loaded incorrectly on that short trailer is what causes the wagging. You had the weight too far rearward on the axles. Even a foot of extra length pushing the trailer axles rearward would have helped the control.