Vibration no one can find

No more than you would see the driveshaft being slightly off.

 

Before you take the driveshafts out I'd check the ride height and adjust it. Make sure you mark where its at now so you can go back if it doesn't do anything.

 

Check this out from an old post.

 

What you describe is unlikely to be caused by a drive shaft.Seriously as Brain and I mention it is harmonic(the wave you describe) caused by slow wear and loosening. A drive shaft balance issue should be progressive and dampened by weight not increased. As you put more strain on your motor (high weight) the vibration is transferred to the frame, when empty you stay out of the torque band and feel less.

Spend your money how you wish, but I would check my motor mounts before paying a shop. Also Brain has a good idea of buying a torque wrench and checking all chassis and suspension bolts.A few hour investment may mysteriously solve it without "fixing" anything.

 

+1 with the harmonic balancer. If it goes bad there will be no visual clue, it will not wobble. The balancing mechanism inside stops working, seizes up.

 

Heavyd is right you wont see anything wrong with the balancer but the viscous fluid inside can go hard or leak out so slowly you wouldn't notice.
I'd also be checking the pinion angles, not sure what the spec is but it would only take a shop 10 mins to check.
Vibrations are a pain in the ____ to chase, have you considered having a vibration analysis done, they are expensive but you've spent a lot already by the sound of it, if its difficult to find the vibration analysis should be able to tell you what speed the vibration causing part is spinning, that narrows the search.

 

i had a 1993 379 pete that had a vibration once it ended up being a bolt sticking out of the block that when i was loaded the torque of the engine would make the bolt hit the exhaust pipe it drove me nuts for 4 months. i know you have a freightliner but maybe check into this also.

 

Talked to my brother in law he said its happend twice on two different trucks and it can be a real bear to locate. He owned two trucks, same as yours.
One was a broken crank, it broke just behind the most forward piston and threw the crank out of balance and the other was the harmonic balance.
Hope this helps, good luck.

 

I agree, The ride height could be set wrong and affecting the driveline angles causing vibration. At least it won't cost anything to check it out.

 

I believe stranger, les and brickhauler are on the money. I once had a vibration problem. Turned out it was the air bag leveler rod. The rod was about an 1/8th inch off messing up the driveline on the u-joints. Kind of like riding with your air dumped, oops. Some are adjustable and some trucks have fixed length rods of different lengths. I heard stories of drivers buying the wrong rod length. Mine was adjustable and the Peterbilt dealer had a special tool (tape measurer I believe ) to set mine right as it was covered under warranty. Rides great now. You can ask a shop for the specs.