Rear strut complete assemblies

That isn't uncommon. The companies make the items they can sell and the front units are more of an interchangeable item than the rear units as those don't get replaced near as many times as the fronts. Plus the rear mounts are not as easy to replace as the fronts. For the rear units you normally break them down and replace only the struts.

They do not offer fully loaded struts for the rear of this car. You will need to separate this coil from the strut your self. Here is how in the diagrams below. Check out the diagrams (below). Please let us know if you need anything else to get the problem fixed.

The brackets are different front to rear. The early rear strut unit holes are 3.25 apart. The fronts are 3.5 and located in different areas on the bracket as well. The front and rear struts also have different spring seats and damping calibration. Plus the rear struts extend through the mount bracket where the fronts are even with it to provide room for the axle and CV joint.

GM changed the strut design mid year on the rear. Take a look at rockauto, they had a few options listed for early and late units.