what is this shake
 

i have a feeling that either my torque arm bushing is bad or my transmission mount is bad too. when i am in first, and get on it a little bit and let off i hear a clunk, feel it too. how could i check what is causing this.
second, when i get on it or when i get about 2500 rpm in third i feel a shake come on under normal driving. when i get on it, it shakes again from 3rd gear till i get off of it, even as im slowing down. im thinking this is a unbalanced driveshaft. how can i tell if it is.
car has a custom reverse split cam, built 4l65e, and 4k stall if that makes a difference.

I would check your trans mount and your u-joints. Remove driveshaft and feel for any binding in u-joints. Also, remove bottom nut of trans mount, jack up trans and see if mount is broken.

Shaking your seat or steering wheel?