06 GT vibration

Has anyone had a problem with vibration when decelerating. I have an 06 GT 5spdM that has a very annoying vibration when delerating in 5th gear from any speed above 45mph. Only had the car 4 weeks, but noticed the vibration on the 2nd day. No vibration when accelerating, coasting in neutral or any gear other than 5th. At least the vibration is not as noticable in any other gear. Push in the clutch - no noticable vibration. The dealer originally said it was normal, but I didn't notice it on the one I test drove. Had it in 3 times so far and emailed Ford (no response). The dealer said they drove another one and it had the same vibration. I didn't buy it and asked if it could have something to do with the 2 piece driveshaft. When he said it was a 1 piece I knew he hadn't ever looked under a Mustang GT. Had the salesman let me drive another new GT, NO vibration so we pulled it into the service area and now they are going to look an mine again.

If anyone else has had a similar problem, do you have a contact at the dealer that fixed it so I can have some additional amunition.

Thanks !

Update:

Left the car at the dealer all day Monday. They connected an electronic engine vibration analizer and reported no abnormal vibration. First order and third order engine vibration during coast down was due to eht engine being bogged down. Talked to the service manager and he asked me if I was lugging the engine. BULL$#%&! The engine turns 1000RPM at 33mph and the vibration is on COAST DOWN from speeds above 45mph. I can add this trip and statement to the list of dealer statements.

When I originally left the car to have the vibration fixed, they said that they drove another one and it had the same vibration. I cannot believe Ford would make a new Mustang with a vibration and I did not notice a vibration on the one I test drove. So I asked if a problem with the 2 piece driveshaft could cause the vibration. The service mechanic argued with me that the car had a 1 piece driveshaft. We went back and forth a few times until he agreed to put the car on a lift to show me. After driving the car into a floor jack that wasn't lowered enough to clear, he raised the lift and then apologized (BECAUSE THE GT HAS A 2 PIECE DRIVESHAFT). When I suggested checking the pinion angles (u-joint angle at transmission), he said there was no adjustment until I showed them the spacers at the center support bracket. He put in some additional washers to change the angle, but no noticeable change in vibration. When I looked under the car over the weekend, the center bracket had been twisted out of alignment when they had torqued the bolts.

THEY SHOULD HAVE KNOWN A 2006 GT HAD A 2 PIECE DRIVESHAFT.
THEY SHOULD HAVE KNOWN HOW TO ADJUST THE DRIVESHAFT ANGLE.
THEY SHOULD HAVE KNOWN HOW TO TORQUE THE BOLTS ON THE CENTER BEARING SUPPORT BRACKET.

I don't know if the vibration is from the driveshaft or possibly from the transmission since it is only during coast down in 5th. I asked the dealer if they had swaped the driveshaft from the one I drove the previous Friday into mine to eliminate the driveshaft as a source and they said that could only be done with Ford's authorization. I had received a reply from my original email to Ford that said the dealer would need to duplicate the problem before they would be able to help. This will just continue to go in circles without fixing the problem. Another email went out to Ford.

Angle should be between 1/2º and 3º. I measured mine at 1.8º horizontal. Lateral was less that 0.2º