Hi James and welcome!

The diff isn't rubber mounted because it doesn't need to be. In the Sierra the diff subframe is mounted in rubber for NVH reasons, not really a consideration in an open sportscar. In such a light car as the Roadster solid mounting removes any play from the drive system, rubber mounts would be under a lot of stress in the more demanding situation (standing starts and more vigorous aceleration).

Having said that there's no real reason not to incorporate rubber mounts if you want to, it's just that you wouldn't notice the advantages in a Roadster and they'd be something that you'd have to keep your eye on from a maintainance point of view.

Cheers

Chris