If you don't get much vibration, so to speak, at high RPM, then it's not likely that the propshaft is out-of-round or off-center. Balance would be OK, too. Likely, it's the somewhat rougher running typical of many engines at idle. You may be trying to idle too slowly, which causes the engine to fire unevenly, and that makes the vibration. You may have to accept an idle that's a few hundred RPM higher to smooth things out.