I agree, if the rear was just dangling and the body supported, you probably aint gonna feel it. I'd be pulling the driveshaft, checking u-joints and tail shaft slop on the trans. My 68 Dart drove me nuts like that. Turned out to be a crack in the rear yoke obviously. New u-joints balanced shaft and everything. Still did it then it finally split and my driveshaft flew out and into the Flint river.