Depending on the idle speed, the response of the body is typical due to the applied torque.

When you shift to reverse, the back end of the vehicle will climb accordingly.

The diff mount might be tired, but it usually doesn't contribute to this phenomena because, although the rubber may sag, it's still relatively rigid.

You ought to check the diff mount anyway because they definitely do get tired due to the constant load on the rubber. I've got some photos in the archives that show the new mount versus an old one.