August 15, 2010
The way that Mustangs are made (quickly), it`s really common for them to be a little sloppy. Usually, if the body looks like it sags, the real problem is how the body panels are welded together, while the subframes are actually level. This is very common for classic Mustangs and especially Fox (`79-`93) Mustangs.
I thought the newer ones were welded more precisely, but it would not surprise me if they still had the occasional slightly off welding job.
Another thing you could check is to see if one or the other of the rear springs is rotated in the perch. Usually there`s a pocket in the perch where the last pigtail of the spring sits. If the spring is not in that pocket, it will sit up higher. I haven`t looked at the rear of our `05 but generally that`s how coil springs mount.