Tundra vibration only when towing?

[Degs98]

I am getting a vibration when I tow trailers. It is a 2004 Trd with low miles. Starting at around 4o mph up to highway speeds my truck vibrates. I can feel it in the seat steering wheel and i can watch my passenger seat vibrate. I have an aluminum 2 place tilt bed magnum trailer and a 21' sled bed 4 place aluminum trailer that I pull. When I first got my truck if i was on fresh blacktop i would get a little pulsation everyother second at slower speeds but I dont notice it as much anymore. I have had 3 different sets of tires and I have towed those trailers with other trucks to not have any problems. My local dealer says its the trailer and they told me to put a ball in the bumper and try it. I wasnt able to do that because the hitch goes into there. I tried it with a used 04 there and same thing. I have towed it with a dodge and a chev and nothing. I called toyotas hotline and they didnt have any ideas so they called the dealer and set up another appointment. The dealer told me if it doesnt do anything without the trailer they werent going to cover it. I understand where they are coming from but at the same time i have warranty and the tow package is from toyota and that is what trucks are made for. Any ideas??

[Only Toyota for me]

I'm assuming you have a u-joint going bad from what it sounds like to me.

How well have you maintained this truck?

Do you grease the driveshaft at least every other oil change?
Has the transmission been serviced?
Diff fluids changed?

Some of this stuff should be done according to TIME or mileage, so even though you have low miles it should still be done. Your truck is 5 years old now, which puts you in the 60k mile towing schedule.

[Degs98]

The dealer had the driveshaft out i believe said u-joints and driveshaft looks fine. Ive only had it about a year and a half and the guy who had it before me took amazing care of it. I havent changed the fluids other then the oil which i run synthetic, and i do grease it yes. It has 42,000 on it so like i said its low miles. The weird thing is that if i put the 4-wheeler in the back of the truck it wont vibrate only when I have a load behind it. That makes everything seem like the trailer but I know it isnt because as i said i have pulled my trailer with other trucks and it wont do it. Im stumped