2001 BMW 325 Million Dollar Question

When excelerating below speeds of 20-25 and hit 20mph, the rear-end vibrates. Best way to discribe it, it sounds like a propeller, but ends exactly at 25 and doesn't come back there after 25. Also, while traveling at normal hwy or road speeds, car makes howling noise that seems to be coming from the rear of car. The vibration comes in while in second gear and into third, but ends in third. I have a slap stick auto trans and haven't tried to leave in first to see if the noise or vibration happens because the RPMs seem to be to high at the speed it starts. I don't want to cause one more issue if you know what I mean. If anyone can help me with this, it would be greatly appreciated. Because, right now, it's just a guessing game.