I did this a couple years ago to my '96 Mystic. It's not too hard but there are a few issues I recall.
The drive shaft attachment at the diff is different. My stock '96 shaft worked fine once I swapped the pinion flange on the diff.
The rear upper brackets require a couple holes that we don't have. Someone told me it's possible to drill the holes and wiggle a U-bolt through them, but I just bolted mine up with the bolts that ARE there, and then welded it.
You'll need '99+ rear brake hoses. You'll also need to do a little re-routing of the hard lines. The stock junction where it splits to go to the other side is an interference point for the upper control arm.
While the whole mess is out of the car is a great time for a diff brace, better bushings, etc.
Any other questions, just ask.