Does it lose any power when it vibrates? Is it a sound or just a feeling? Something moving around when it squats, or holding speed on a steep hill shows the same?
Could be a dead exhaust bolt/gasket or similar, otherwise I would check for stupidity like swaybar links near the exhaust, wheel nuts etc.
If not something little, after you get home and its doing it jack up the rear by the diff and spin the wheels by hand and listen/feel.