Truck was always great to drive
Ford F150 Crew Cab V8 with 3.31 rear diff. 4x4
Now... We have a new thing;
slight intermittent vibration around 40 - 65 mph lasts about 2 secs. (about every 5 - 6 sec it starts again)
As if you run over a rumble set of strips, you can hear it
You can feel it thought the car, steering and through the seat I sit in.

Dealer can feel / hear it as I drove it with a technician
They cannot locate and tell it is ok, drive till it gets worse but here is the bill for diagnostics

Only under load, maintain speed or on a slight incline it is most noticeable
Yes, I can feel it at lower speeds as well as long as its a smooth road you are travelling on

Tires are all rebalanced, rotated
New front brakes
Had an alignment, all linkages are good.
It is drivetrain related, only while driving...

Any thoughts anyone?
Thanks!

 

I have a similar vibration and dealer replaced the driveshaft which did nothing. Another dealer tried different tires/wheels, no change. Vibration frequency is similar to an out of balance driveshaft or bad u-joint. Dealer gave up.
I was poking around under the truck and noticed that there are a couple cast iron donut shaped pieces attached to the rear of transmission. Only function I can see is they are there to dampen a harmonic vibration in the transmission. Adding weight changes the resonant frequency. Maybe not enough or too much weight at that location? Haven't experimented yet.

 

Do you trust the shop that balanced the tires. I had brand new tires installed a had a vibration for about 5 weeks. They just didn’t get one balanced correctly the first time.

 

Sounds like what I feel in mine. I need to do a transmission fluid change and I'm hoping it fixes it.