lsc91

I just bought a 00 cherokee with a blown motor and replaced it with one from a salvage yard. When its running it has a vibration and when you run it at 1200 rpm it rattles and vibrates badly until about 200 rpms then it smooths out. This is when it in park and through the gears. Just wondering if anyone has any ideas on what to look for. When I put the motor in I replaced the motor mounts and I made sure the torque converter was tight. Thanks for your input.