How about just revving the engine into that range with the trans in neutral, or with the clutch depressed? If either of those situations cause a shake, it's definitely engine-related. It's possible the balancer ring could have slipped, or maybe it's gotten a little crossed up. Use a strobe from a timing light, and see if the balancer is wobbling. By switching to different spark plugs with the trigger lead, you can inspect the damper all the way around. A delay-flash light would serve the same purpose.
Jerry
