I just did mine on Saturday and had the same problem. I put a 12 inch long box end wrench on the nut and put a floor lack underneath it to break it loose, then hit it with the impact. If you drop the spare and remove both rear wheels, you end up with a ton of space to work in, and can access the nut with a deep socket and extension.
FWIW, I removed the factory shocks. It was the first time they've been off since 2012, and the nut was on the spring side.
FWIW, I removed the factory shocks. It was the first time they've been off since 2012, and the nut was on the spring side.