TL;DR - How can I isolate whether a vibration is being caused by a bad driveshaft or a problem with the rear differential?
Apologies for the long story, but here goes...
I recently acquired a 2007 Chrysler Pacifica Touring (4.0L AWD) with 185,000 miles from a small independent dealership a little more than an hour away from me.
Over the first week of ownership the car developed a vibration. At first it was a little between 35-45mph but quickly worsened in intensity. On the 9th day I took it to the dealership to demonstrate the issue, but as their in-house mechanic isn't a driveline specialist and they know I'm not local, they suggested I take it to a shop local to me for diagnosis. On my way home, the driveshaft broke off where it mated with the rear differential.
Luckily this was less than 1000 feet from a repair shop and they removed the broken driveshaft and noted a leak in the rear diff. I agreed to drive the car to the dealership to obtain a loaner. During the drive, the car now vibrated between 50-60mph.
The dealership covered the problem under warranty, however, because Chrysler no longer stocks this part the dealer sourced what they said was a remanufactured unit after a 30 day wait for availability. The dealer says they had the local Aamco shop install it for them.
After getting the car back, the vibration has returned, this time between 30-40mph when I accelerate or go uphill and initially we were back to "have your shop diagnose and we'll work it out".
The first place I took it to specialized in transmission work, they said the driveshaft was "out of balance" and "needed rebalancing". They said the rear diff did not appear to be leaking and suggested I replace the tires as they were two different brands. They did not charge me for the diagnosis and I do not believe they removed the driveshaft.
I went ahead and put four new tires on the car and had an alignment done but there was no change in behavior.
I wanted a second opinion and the second shop claimed that the driveshaft being out of balance was B.S. and it was the rear differential -- until they removed the driveshaft. Now the vibration is completely gone and I have verified this at speeds of up to 85mph. The second shop says it could be the driveshaft -- but it could still be the rear diff. The second shop said that because they couldn't definitively isolate the problem without actually taking things apart further, they didn't want to haggle with the dealership over parts, price, etc. since it's a warranty job. They did charge me a small amount for the labor of removing the shaft.
Ultimately, both shops point at the driveshaft as the culprit and the next step is to go back to the dealer and have their regular Aamco guy sort it out.
Because of the distance to the dealership, I am trying to minimize the number of trips there and back, being away from work, etc.
But I'm having difficulty reconciling what I've been told with my experiences, which neither shop was willing or able to address:
On the one hand, I find it hard to believe it's the driveshaft since it's essentially the same vibration I had before. Having seen both the original and replacement driveshafts, they looked different so I don't think the old shaft was simply reinstalled (unless someone went to great pains to sand it, paint it, and put salvage yard style markings on it). How could two different driveshafts be the cause of the same symptoms?
On the other hand, if it's the differential, why did I have vibration the first time the driveshaft was removed, but not the second time?
And lastly, are there other steps I can take to try and further isolate the likely culprit for the vibration?
A couple of additional observations:
The vibration feels like it's coming from the rear, the sensation is more in my butt when I'm driving and not at all in the steering wheel. I can also feel it through the center console.
The vibration is also consistent regardless of gear, if I put it in Autostick I can shift all the way from 1st to 6th and it's still there.
While doing another round of calls to local shops I found one place that was willing to rebalance the driveshaft for $250 but also talked to a third shop that was interested enough in my story to look at it for free.
Like the second shop, the third shop initially said over the phone it was extremely unlikely to be the driveshaft but upon actually diagnosing the vehicle concluded it the shaft is in fact the culprit.
Apparently (I have not verified this personally because the shaft was reinstalled for diagnosis) there is some play in the u-joint but you have to hold the shaft a particular way to feel it.
Naturally, the u-joint could theoretically be replaced but the components are no longer stocked so we are back to finding another replacement driveshaft.