The HD differential was found in teh 98+ GTP'*, 98+ Bonneville SSEi'*, 98+ Park Ave Ultra'* and 98+ Regal GS'*. All the same.
If the car is not the supercharged model of these vehicles it will be a different gearing ratio and size. The differential found in the 97 model year was revised slightly for the 98+ years. You will want to concentrate on a 98+.
The HD diff on the 97 and 98+ are about the same and will work as long as you use the output shaft. The splines are what'* different. But like Bill said, you can't take the 97 HD diff and stick it in a 98+ without the output shaft.
Actually Dan, it has been done. I read about it on another forum. IIRC it was tough, but possible.
The differences make it tough though and when I look for them, I personally skip the 97'* for the reasoning that none of the vendors accept them as cores, they are different from all other years, it'* simply not worth the added hassle.