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.

 

To check the bushings get under it and look at the bushings.

It seems like a common complaint to have a vibration in third. It may be because everything is turing the same speed in that gear. With a 1:1 ratio the engine, trans, and driveshaft are all spinning the same speed.

To check if the driveshaft is out of balance get up to speed in a gear other than third. Do this after you get the mounts checked and replaced.

Bad mounts can throw the driveline angle off also which will cause a vibration.