After changing the struts on my 97 Impreza I took it a shop for alignment.  The left rear adjustment nut was goofed up.  Looks like someone remove it with a vice grip.  The shop wanted to charge me $150 to remove/replace the nut "we don't know how long it's going to take".

 

After spending all Sunday on the strut removal I wasn't about to spend $150 for the removal of one nut.  I brought it home, jacked it up and removed the nut.  The bolt is fine. So...

 

Is there anything specific about that nut that I should get it from the dealer?  I have a flanged nut in my garage that threads on fine but just wasn't sure if there is a "hardness" issue or something that I should not use whatever I have hanging around

Probably talking about the rear inner nut?  Nothing special other than it's a locking flange nut.  If the one you have is corrosion resistant it should be fine... or just paint it afterwards.

 

It's the nut on the rear toe-in bolt.  I don't see anything special about it.  I have a flanged nut that I just took off the top mount of the old strut that is in very good shape as it was inside the car.  Thinking I can just use that and torque it down.

see attached photo

Yes, that's the one I was thinking about.  The strut nut should work fine.  $150 to use a wrench

Most suspension bolts and nuts on these are 10.9 metric grade. As long as it threads on it will be fine.

 

Typically before I take a car for alignment I loosen and/or remove any bolts that I know they will need to adjust and put antiseize on them.