My '08 with 44K miles, when going downhill at 40 mph on a moderate steep grade, sounded like it auto downshifted and jumped up to 2K rpm and I felt vibrations. It doesn't do that when descending on a lesser grade and at higher speeds. Any thoughts?

Normal condition, the transmission is designed to downshift while descending, this helps control the vehicle. It usually happens after you apply the brakes several times. As for vibrations.... might be normal NVH, or?