Boost-Me

Hey guys I have an odd vibration that just started and has me scared to drive my car. It started earlier when I pulled out and took my car through 9000 in first and second gear before I let out. Moments before shifting to 3rd is when the noise and vibration started.
Now every time I drive the car today, once I get to around 40 MPH I can feel the vibration but I only feel it when I press the clutch or if I let off the gas and coast. I was letting a mechanic ride with me so I could show him and while he was with me it got a lot worse and now I don't even want to drive it cause we are afraid it is about to come apart.
Anyone have a guess on what could be causing it? He thinks it could be the clutch or trans but is leaning more towards the clutch. It feels more like a differential, axle, or driveshaft problem to me since it starts at a certain speed.
I'm handy and can do most work myself, just not good at figuring out problems so before I start throwing parts at it I wont a good place to start.