this will sound crazy, but unhook the trans, slide it back, support the rear of the engine and fire the engine up, see if it still shakes, if not, you know its in the trans/flexplate/converter.
Great idea but unfortunately it won't work cause the starter ring gear is attached to the torque converter and the starter bolts to the trans housing. Unbolt the trans and converter and there's no way to start it short of installing a flywheel and manual trans bellhousing, which may not be a bad idea. At least that'd rule out the trans and converter