I went through a very frustrating drivline vibration diagnosis not that long ago.....You really need to check everything, and check it carefully and with great detail.
I was working on a VERY highly restored vehicle that paid exceptional attention to every detail concerning correct parts and markings and so on....and it all really did appear perfect.
As the job progressed here is what I found, in steps, after finding and fixing each items along the way to eliminate parts as the process played out.
The driveshaft tube was slightly oval shape, more at one end than the other....no signs of ever being hit or damaged. This was a bad tube from the get go and nobody noticed....it was out of round enough to see with the naked eye.
The rear driveshaft yolk was welded on out of round and this was not the oval side of the tube.
The drivesahft was not properly balanced....It had a HUGE amount of weights welded in place, on both ends. This alone should have given the driveshaft maker, or balancer, a major clue that something was not right.
You might be surprised to find how many "good" shops don't really have a good balancing machine, or they have one that is out of calibration, or they have an operator who does not really know what they are doing beyond the basic movements.
Talk with the person who will do the balancing to get a feel for their level of understanding.
Side note: I once dealt with a big driveline shop (in Utah) that told me their balancing machine was so sensitive that if they balanced the shaft, took it off the machine and put it back on the machine, the driveshaft would be out of balance...........I was broken down on the road, far from home, and stuck with this shop. The shaft they built was shit by the way, but it got me home.
The slip yolk was an old piece that was refurbed, but it had taken a hit at some point and was no longer true.
The rear driveshaft yolk was new, and....wait for it.......crap out of the box. Who ever heard of that happening in this day?....Oh yeah, pretty much everyone who works on vehicles because so much is crap out of the box today.
The differential yolk was worn (actually ground by someone) by a large number, like 0.040" or so, between the two cap retaining humps. This allowed the U-joint to move back and forth enough that it may, or may not, be placed in the proper postion.
The reason for the big gap on the rear differential yolk came at the hands of someone trying to fix a non-issue they did not understand. This particular car used a rear U-joint with different sized caps and someone installed a U-joint using big caps on both ends rather than big on the driveshaft and smaller on the differential yolk.....so they ground things to fit instead of sourcing the proper U-joint......But hey! it has the correct paint marks so it was right.
The front U-joint, new parts from auto parts store, was out of round out of the box....but installed anyway. This one I can forgive because we should be able to expect a good U-joint when purchased new....but we can't anymore.
I specifically use Spicer U-joints because they have proven to be better, but they are not perfect so verify them too.
Precision brand U-joints stink a lot and was the part found to be faulty in this instance.
All of these items gave rise to a vibration that was difficult to find and even more difficult to diagnose due to so many items being wrong at once.
Check everything and do NOT assume any parts are correct until they have been verified correct.
