When I towed my new boat home for the first time I almost lost it within 5 miles on the pickup point. Problem.... well a 1500 Chev towing 11000# plus was one of the problems, however, two things we discovered were:
1. Boat was to far back on the trailer and there was hardly any tongue weight
2. The outdrives would sometimes swing back and fourth causing a swaying effect.
Moving the boat forward and tieing off the drives helped sort out the problem. Buying the dually three years later solved it.