Weird Vibration!

It sounds much harder than it should be. I have a couple manual 3 spd tannys at home, I unbolted the speedo gear, and the 5 bolts around the tail shaft housing, and the tail shaft housing slid right off with no problem. I don't see why it would be different for a four spd, I should be able to do this while it is in the car. I don't see why I can't. Whats soo different on a 4 spd?