Major rear-end clunking after gear change

[Stokestack]

Hi all.

I recently had 3.73s put into my '96 Cobra, and now there's a boatload of noise coming from the rear end. Not only does it clunk on shifts, but often when going around turns it sounds like I have a trunk full of golf balls. Seriously.

It sounds like something is shifting significantly when the drivetrain is engaged or disengaged.

Some people have suggested that the installer forgot to add friction modifier. Is that likely to account for such a panoply of noises? And what's the procedure for adding modifier (including removing some gear oil to make room, and where is the filling location)?

Thanks!

Gavin