I have just been informed my rear shocks have reached the end of life.
One is starting to rot and the other is seriously worn.

Is this something I can find aftermarket ( better price ) . or just let Midas replace them

Yeah, you can get the auto-level struts in the aftermarket. Monroe makes a nice set, at least for the newer ones, but I'd assume they'd make a set for yours too.

If you're willing to do the labor, I'm sure you can do it for cheaper than Midas. Of course, you'd still have to get it aligned. Hardest part about doing struts is getting those strut-to-knuckle bolts and nuts off. It takes a good impact wrench, or a good breaker bar, impact socket, and long pipe. Or both.

Here'* my two cents,

After 225,000 miles i decided to have all 4 struts replaced by Midas, also asked them to replace the muffler. They ended up doing several hundred dollars worth of damage in the process, so take it to a trusted mechanic. I asked them for Monroes replacement struts, they sold me their "just as good" part, the LF failed 2 years later. So i replaced both fronts with new ones, did the labor myself. Its very doable with two people, took one afternoons time. I have the Pontiac factory shop manual with complete instructions, PM me if you want a copy. Once you learn the tricks in doing the first one, the rest are a breeze.

Those big bolts werent that bad, just use some penetrating oil and some muscles