Rear End problems ....01 Trans Am ?
When you say the left is closer to the tire do you mean horizontally or vertically? If it is horizontally it means your rear may be out of alignment with the front. If its vertically then you may have some sagging on that side.
As for the rumbling, is it new or has it always done it? I'd start by going underneath and retorque everything down (suspension, u-joints, etc), check pinion angles (you can even download an app for your phone to do this), test it, and then start worrying about the rear internals and the drive shaft. Basically start at the cheapest and work your way up to possibly having to fix the rear.

