ziggy75,
Hmmm....
Rebuilding a differential is not a beginner's chore. The pinion depth needs to be exact, and the tooth engagement pattern takes some labor to get correct.
Here's a sample of "how-to":
1998 Ford Mustang Cobra Budget Rearend Rebuild - MM&FF
The fancy limited slip isn't needed for normal street use; you can use the standard one from Ford perfectly well.
If you search on "mustang 8.8 axle rebuild" you'll see a bunch of hits describing how other folks get it done.
Then, you decide. Is it better to have a pro do this job? I can't answer that one for you - I have done a couple and they didn't always turn out perfect - perhaps yours will!
If it was easy, someone would have already done it LOL
Tom