Rear disks were fitted as standard to the 2litre DOC Sierra GLSI & XR4i.
Abs was also available as an option on other models, these would have also been fitted with rear disks.
There is nothing preventing you from fitting rear disks if you are able find the components needed.
If memory serves me right. You will need Rear uprights made to suit disks Or adapters such as the ones 3ge sell.
I am not sure if the drive shafts need changing. Rear disk Sierras were fitted with the Lobro type shafts.
