I know there are tons of threads on this and ive spent hours reading through them. but there are a few differences to mine.

2005 F350 6.0l regular cab long box. 4" lift on Duratec 33". Wheels balanced, driveshafts balanced, all u-joints checked and 1 was replaced (made no noticeable difference), all new brakes and rotors recently(due to wear)

When im not towing, vibration is noticeable between 60-65, but its only slight and i can live with it. Its felt in the seat and not the wheel.

There is also a very low grinding/pulsing feeling that happens as i am slowing down from highway speeds starting at around 10 mph until i completely stop. Its not the brakes and feels driveline to me. Can only be felt at these very low speeds while decelerating/coasting. Feel it in the seat and not the wheel. Not the end of the world but i have a feeling its related to my high speed issues. It is not felt on acceleration and i dont feel it when im moving around parking lots/etc. ONLY when decelerating from highway speeds.

When im towing the vibration randomly gets really bad right around 65mph the whole truck shakes and its very uncomfortable. It doesnt matter if im on the gas or not, hill or no hill. If i slow down to 60 it goes away. It is very rhythmic with road speed (3 pulses with a slight pause)

Any ideas? Been doing a lot more towing lately and its bugging me. I keep falling back on tires, but not quite ready to bite the bullet on new tires as the ones on it still have tons of life left.

 

Any change after rotatating tires? You said u joints checked but they are cheap so may be good to replace

 

I have rotated the tires a few times and there is no noticeable difference.

The shop that balanced the driveshaft told me there is nothing wrong with the rest of the U-joints. I may still end up doing them though.

 

After what you describe it feels like tires. They may have a bad wear pattern from who knows what. I know I had a set of tires wear poorly, for no particular reason that I could find, that caused a weird vibration at a certain low speeds. They also made a weird sound at high speeds. Do you have a friend you can trade tires with for a day? Just swap the wheels out and see what happens. That way you don't have to buy new tires if it doesn't help.

 

I had a balancing weight fall off one of my tires last year; found it in my driveway so looked at my tires and saw a shadow where the weight used to be so stuck it back on. Only use the truck for plowing, so didn't notice any issues with it off (don't think it was off long).

Probably not your issue, especially at low speeds, but you never know.

Try putting your spare on each of the four tires to see if that makes any difference

 

Sounds like u-joints to me also, or at the very least the driveshaft slip joint needs to be greased with the special Ford grease.

 

Your tires are too aggressive to notice a slipped belt, and you really would feel a bad belt more than how you describe. Chopped up tires (from bad shocks) will be hard to see as well.

I'd look at the axles. You could have a bad wheel bearing (front and/or rear) or even an issue in the gears. If you really want to dive into this, pop the differential covers, clean off the gears really well, inspect the bearings, and then put gear paint on to check the drive and coast patterns.

Have you given her the good ole shake down? 12 and 6, 9 and 3, and then stick a pry bar underneath the tire and lift up (while suspension is unloaded) to check the ball joints.