Havent been on here a while, good to be back.
I bought a 14' jon boat this past spring and when towing the 1,000 lb. boat & trialer I get a strong in and out vibration at highway speeds 60 mph and over, other than that I dont even know its behind me. This is the first thing I have towed with my truck, but I have loaded the crap out of the bed and never had any vibration issues. My truck is completely stock and I had the driveshaft rebalanced about a year ago after my mechanic noticed a missing weight, not sure I noticed it or it made any differance though he just happened to notice it replacing my diff fluid and checking u-joints.
I would think if it were the weight of the boat causing the driveshaft angles to be out it would have vibrated whith a fully loaded bed too, Ive had it squatting pretty low but not overkill.
Not sure if the trailer tires (tubeless) have been or need balancing or not. Seems to me this might be the issue but I wanted to check with the forum before I start shelling out cash.