Vibration in driveshaft?

Motor mounts

 

Checked the ride height?

 

Driveshaft carrier

 

Carrier bearings?

 

I used to work for a Kenworth dealer as a mechanic. They sold a customer 2 identical tandem logging
trucks. One of the trucks had a buzzing vibration that started at 50mph and got worse up to 65mph.
The first thing I checked was all the drive-line and component angles. Both were exactly the same and
correct according to the build sheet. I swapped all 3 drive shafts between the two trucks and took them
for a test drive. The truck with the vibration still vibrated and the other with no vibration still had no vibration.
Next I made a 1/2" shim to go between the carrier bearing and bracket, took it for a test drive and no change so I took it back out. The next thing I tried was rolling the rear axle back and increasing the U-joint working angle. I added four 5/8" washers between the top torque rod and the axle housing. Probably
two or three degree change in angle. I took it for a test drive and the vibration was completely gone.
Why would the truck need a different angle when its twin had no problem?

From my dealership training there are two distinctly different vibrations and a lot of people are
not good at describing them. There is a high frequency vibration usually from engine drive-train
components or drive shafts. There is a lower frequency vibration from the wheels tires and drums.

 

Another vibration I dealt with at the dealership was a new Kenworth cab over with a 4mg series Cat 425Hp turned up of course. From the day the truck was new the owner complained about a vibration. I went for a ride with the truck loaded, 8 axle lumber hauler, what he described as a vibration I would call a rough running engine under load. All the top end adjustments were perfect and he took it to a Cat truck shop in a city seven hours drive away and the said the pump and injectors were not the problem.
He stopped complaining and I guess got used to it. A couple years later the truck came to our shop to
have larger fuel tanks installed. I took the two 100 gallon tanks off and added brackets and installed a
pair of 150 gallon tanks. Kenworth uses #10 SAE fittings and line for fuel suction with a #10 Tee to
connect to the tanks. I had to make up a short extension for both suction and return. When I went to
reconnect the fuel lines I found that the factory had used a tee with two #10 flare fittings going to the tanks but third connection was 1/2" pipe thread. They had threaded the #10 line that went to the primary filter
onto a 1/2" pipe fitting which is close to the same thread.
A couple days later the owner came into the shop with a big smile and wanted to know what I did to fix
the vibration as the engine had never ran so well.
The only thing I could think of was there must have been air getting drawn into through the threads and it was causing uneven fuel delivery to the injectors.

 

So if i get this correct....accelerating nothing, decelerating nothing.....just steady speed?
What happens if you push in the clutch while its "doing it"? What happens if you put it in neutral and let it coast when its doing it? How sloppy is your shifter?

 

Could've thrown a weight off the driveshaft.