This is a photo of half of the equalizer that is nearest the rear brakes. Missing from the photo is the portion of the equalizer that is nearest the truck cab.
The nut that won't spin is a 12 MM elongated appearing nut on the threaded portion of the cable. At the right side of the photo, the threading of the cable becomes square stock -- and that is where I attach the vice grips. An open end 12 MM wrench fits the nut, yet I can't get the nut to spin.
https://gyazo.com/6bc97acaa72d6cc293b5833750c4ba97