Welcome!
I'd not be as concerned about '03 vs '04 as ensuring that you have a sound differential.
Our '03 is a super car, aside from the differential issue. Sure the '04 has a high tech V6, but the 3.2 is a fine little engine in its own right.
While it may be true that most have had no difficulties, a search will reveal a significant number of folks have experienced problems with their differential (within the V forum as well). This would be my primary concern if I was shopping for another.
I'm not familiar with the rear end chattering when reversing, but am all too familiar with the classic differential moan. After the car has been driven for 25 - 30 miles or so, listen for a moan or whine at 35 - 40mph or thereabouts under light throttle or trailing throttle. If you hear a moan, and it will be loud enough to identify, I'd steer clear of that particular car. True, apparently some moan & keep on trucking. Some get progressively worse and fail. Failure at speed is no fun...
