I get the "clunk" in my 96gt from time to time. It happens when I am shifting into 1st from neutral from a stop. I work at Jiffy Lube and one day last week I pulled it in after we closed and went down underneath it to change my fuel filter....while I was down there I checked for play in the u-joints. They were good but the driveshaft itself has play in it. Feels like the ring and pinions need shimming or something. I guess it is what's described in the above post.

 

You got it....shim away.

 

replace them rear gears from the factory while your back there.