Car began to make whining noise now clunks and jerks. Rear differential? Tranny?

What is your question?

The car began to make a whining noise when I accelerate, no noticeable noise when coasting in gear, and a winding down noise when I apply the brakes. After about a week of virtually no driving I had to drive 13 miles, sounded like something broke or fell off under car, some rattling noises, then began making clunking noise and felt jerky/locking up. No visual broken parts under car. Could it be the rear differential? What might be the most likely culprit, the place to begin my process of elimination?