i am going to tell you this, although you might not beleive it. a guy sold me a 73 or 74 pickup with a clutch chatter in when going in reverse,he gave me a good deal because he was sure that it needed clutch work, and it did shake the truck[i think reverse was geared a little higher than first]. anyway, i did a tuneup first when i got some spare time, plugs, wires, distributer cap, ect. and low and behold, no more chatter in reverse, and i didn't touch the clutch. i can't honestly explain why the truck would shake like it did, but some aspect of the tuneup fixed it. hope this helps. good luck
