Remove rear strut nut
 

How can I remove the rear strut nut that holds the spring in. The top broke from trying to hold it still in a vise. Please help! I need this done by morning

Does it have an allen head? Most you're able to put an allen wrench in it to stop it from spinning and then twist the nut off with a normal wrench.

Loosen it while the assembly is still on the car..you're talking about the top middle one, yes?

Vise grips on the shaft, through the spring. This is one of those juobs where air tools are really needed.

I put the threaded part in a vise, then moved the nut to the top. And I couldn't move it anymore, so I used a nut buster to break it off. Then I used a hack saw to take of the too 3-4 damaged threads. (There was plenty if extra thread)
I used one of the 14's from the front to replace the nut

I had that problem aswell when we installed the air suspension. Put the vice grips on the actual shaft that retracts an impact the nut off the top.

Putting a vise grip on the shaft can ruin the shock. If your replacing the shock, by all means grab it however you can. If your not, only grab the shock shaft up as close as possible to the upper mount to prevent damage to any part of the shaft that enters the shock housing when it's compressed. Even the slightest mark on the shaft will damage the seal in the shock causing it to lose its nitrogen charge and leak oil.

I used a nut buster and bought a new nut:)