My apologies in advance for the length of this post: I have a 26' boat (around 8500 lb) that I've towed for years with a Toyota Tundra. Never had a towing problem. This winter I traded it in for a Ram 2500 6.4L. When I picked up my boat from winter storage I noticed a definite vibration at around 60 mph. I could feel it in my seat and the steering wheel. Since the trailer tires were pretty old, I replaced all four (tandem axles) with new radials and had them balanced. Thought that would solve the problem. Nope. Then I took my Ram to a tire shop and had them check all 4 tires for balance. The rears were off a little. I thought that would solve the problem. Nope. I went to a trailer shop and they said I should have my tongue weight checked (for $160). That didn't make sense to me since I never had problems with the Tundra, which didn't have the towing capacity as my new Ram. I do know that the Tundra had leaf rear springs while the Ram has coils. I've played around adjusting the height of the hitch. No change. At present the trailer is nearly dead level while hooked up to my Ram. I've checked the drop in the Ram with and without the trailer attached. It drops around 1 1/4". The front rises maybe an inch. Not significant as far as I can tell. I'm stumped. Nothing's changed with the boat/trailer in the past 14 years, except new tires. The only thing new is the Ram. It seems to tow fine up until around 57 - 60 mph. Slower or faster than that is fine. One more thing; I don't have any vibration with the truck when I'm not pulling the boat, regardless of speed. Any suggestions? Thanks!