Help Requested - 2006 Cayenne S Drive Train Issue

[chrishill555]

2006 Cayenne S with 110k miles.  Two weeks ago the drive train started to have a mild vibration on release of accelerator.  It was very minor and I just assumed that it was the tires.   All 4 tires needed replacing, so it was not a psychotic thought.  Ironically, a couple of days later, the driver's side rear tire blows.  Car is towed on a flatbed to my house.  Because I needed to get the car to the tire place, i replaced the two rear wheels with two wheels from my 2005 CTT.  My wife drove the car for 3 or 4 days while I was traveling.  When I returned, the noise had gotten worse.  Really bad when you are at about 30+ mph and then release the accelerator.  It had a loud whine.  I take it to my local shop who does work for me when it is beyond my level.  They think it is the transfer case and recommend taking it to the dealer because they have the equipment required to enable a new transfer case to be joined to the drivetrain.  I take it to the dealer.  Of course they say it is the transfer case and will charge me $4500 to replace it.  They tell me that the transfer case is caused by having the wrong size tires on the front.  So, at this point, I think they are possibly full of crap because the two tires on the front at the original ones, but the ones on the back are from my CTT.   The front tires are 275/40R20 and the rear ones were 255/50R19.  Diameter-wise, they were approximately .3" different.  From what I read, that may be enough to cause a problem with a transfer case.  But to me, it is not conclusive.  They convinced me that I caused it with the different tire sizes being on the car and I approve the repair.  I take them the original wheels with new rubber and they do the repair.  I then pick the car up a few days later with the new transfer case.  I drive 1/4 mile from the dealership and on first deceleration, the car makes the same exact sound.   Maybe not quite as loud, but basically it still has the exact same problem as when I brought it in.  So, I take the car back to the dealership and yell and rant and they say they are going to look at it again.  Sorry for this being so long, but I needed to tell the whole story.  I am not sure where to go from here.  Should I take the car somewhere else?  Can I assume that the problem was not the transfer case to begin with.  Any suggestions, opinions and flames requested.   

[wizard]

Differential? You may want to check your fluid level as the next step.

[lewisweller]

Check the flex disc and Propshaft centre bearing for movement.