When I was 17, my 544 had a very bad vibration as well. I was a decent backyard mechanic, but still developing some skills. As I recall, removing the top 2 tranny bolts was a bitch, so I had to cut the tunnel on both sides to gain access. I too thought about removing the engine to complete the task, but was talked out of it by my father. Anyway, after replacing the clutch & pressure plate (but didn't have the $ to resurface the flywheel) then reinstalling, the chatter was still there. Removed everything once again, I was getting pretty good at it by this point and could have it all out in under an hour. Thought the 3rd time was going to be a charm. Then I was encouraged to believe the crank could be the culprit. What did I know, I was a teenager. Bought another engine, with my parents help of course, slipped it in and it seemed to do the trick. My thoughts are that the input shaft on the tranny while rotating and in gear caused some centrifugal imbalance on the flywheel and its rotating mass. Lesson learned and I sold it within a month after fixing it.