Cossie rears are all the same. Be it RWD or 4x4. However, the bottom mounts on a standard (non-Cossie) RWD Sierra is about 8mm wider. Can be modded ofcourse by just grinding of 4mm on both sides of the bush and AFAIK standard Cossie shocks are shorter because a Cossie was lower compared to standard Sierra's.
Fronts are very different. AFAIK the 4x4 shocks are shorter than the 2wd versions because of the hub that is also very different on both models.
Not sure about the diameter differences.