Just got car, question about diff noise

Hey, after finding a very very tidy s12 for sale near my house I went to check it out. The car is immaculate inside and out, the motor is a 2.0 not sure which type, Fuel inj. but it has done 280000, which seems like a lot.....but the car just doesnt look like it has done this amount of K's. There are no signs of a respray as all original stickers are on it and they have faded with time. I took it for a drive and upon initial take off it shudders in the rear end, as if from the diff. Does anyone have any idea on price for a new diff in Australia? Is it possible to interchange the diffs with another unit? And does anyone else have any other ideas as to what this could be? The car is an Automatic and the shudder goes away once the car is moving, so I hope someone can give me a bit of an idea.
Thanks in Advance

You can take any diff out of a straight axle (not independant suspension) and it will swap right in. I don't think it's your diff causing the shudder though, check your u-joints for wear and those rubber bushings in and around your rear end because both can cause a shudder. When diffs start to go they growl or grind and the noise usually increases with speed, Usually. I'de check everything else first as Its hard to kill a differential with a 2.0l and an automatic transmission.

280,000 isn't excessive for an early model one, my 1983 build has 445,000 on it. Is the car a live axle or a IRS (easy to tell from the brakes, a live axle has rear drums and the IRS has discs). Either way the diffs aren't too expensive, i have a spare of both sitting at home right now if you are nearby.
Not sure what the problem could be, maybe the rubber thingo that holds the driveshaft CV joint, it's kinda soft and crappy looking...who knows.

yeah, it sounds like a centre bearing, like what nebs is talking about.