My 2001 F150 has 92,000 miles on it. There is a slight vibration that I feel through the seat and steering wheel when pulling a trailor. One trailor is an 18 ft lowboy with a 35 hp tractor and the other is a 14 ft lowboy with a 4-wheeler. That is not much weight for a truck to pull. So far I have done:
1. Checked the balance of the driveshaft
2. Checked the U-joints
3. Checked the balance of the tires
4. Rotated the tires
5. Checked the shocks
Also the truck drives smooth when not pulling a trailor. Both trailors pull fine when hooked to other trucks.
This site has such knowledgeable Ford owners I thought I would ask for help. Thank you.

 

check the ball joints, tie-rods, steering box, idler arm, id also check your wheel bearings. If these havent been changed might be a good idea. Id also recommend diff fluid change if that hasnt been done lately.

you listed the more obvious things to check so now its time to get into the ackward causes

 

Yea, I'd say the rear gear oil. It does break down over time. Vibration usually occurs @ high speed, like 45 -65 mph. Mine went bad just over a 100,000 miles.

 

Thanks for the help. I will be changing the rear gear oil next.

 

Have you checked the balance of the trailer tires? Any vibration in them would be transferred into the hitch and your truck.

 

I took my truck to the Ford dealership today. After checking out the truck the mechanic told me on some trucks there is a slight vibration when carrying a load. He said the angle of the driveshaft entering the rearend changes when a load moves the bed down. He said there was nothing wrong with my truck. I donot know about this it is just what he told me.