shudder when decelerating. Help!

[PlanZero]

I just purchased a bone stock 97 tsi awd with a rebuilt automatic transmission (600 mi ago). I drove her two hours home with no problems and locally for the last few days. Today I took a short trip on the highway. She boosted fine but when I let off the gas to slow down I heard and felt a horrible shudder from the rear end (I think). After I got off the highway I boosted up a hill and when I came to a stop sign, the same sound happened. I checked the rear diff in a parking lot and oil immediately came out of the plug hole so I know it's full. Didn't have a chance to check the transfer case fluid yet, but I assume its full as well. Brakes are newer, no sounds of bad wheel hubs, etc. Also the axles look OK, no boot rips. When I took it for a test drive the PO said he didn't drive it for awhile and as I backed out of his driveway I heard a clacking sound from the rear( I think) but only that one time. I had what I thought were transmission probs on my 420 and turned out to be the tps. Like I said this car is bone stock any help?[DOUBLEPOST=1410365703][/DOUBLEPOST]Update: this only happens from decelerating from speeds above 80mph. Happened again today and almost sounds like a metallic scraping . I pulled over and peeked underneath and the downpipe from the flex back to the cat is split open, also the muffler is detaching. I'm assuming the cat is shot as well. Could this cause this?

 

[PieEyedPiper]

Exhaust noises from a split open downpipe and falling off muffler are sure to produce some interesting sounds, especially in a tall gear coming off the highway.
It's certainly possible that what you hear is related to that but it might be in your best interests to do some basic tests on the drivetrain and maybe do a fluid change.

"Do the standard tests:

1) lift one front wheel and try to turn it with the car in gear: you shouldn't be able to ... if you can, then either the front or center diff is gone.

2) lift one rear wheel are try to turn it with the car in gear (and the parking brake off, of course): you shouldn't ... if you can, then either the rear or center is gone.

Note that these tests assume that you have not snapped an axle. If you aren't sure, check them all first.

When doing these tests, turn the wheel slowly but with some strength. There will be some slop, but if you can rotate the wheel more than 90 degrees, then the test was clearly failed. (Note to the confused: "failed" = wheel can be turned.)

If the car fails #1 but passes #2, it's the front diff.

If it fails both, it's the center.

If it passes #1 but fails #2, it's the rear.

- Jtoby"