You'll need to use the tension between upper and lower A-arms to help pull apart the lower ball joint, so must do this earlier in the process while they're still connected. Remove the cotter pin on the lower ball joint and back off the nut until it's flush with the end of the threaded stud. Place a jack or block under the A-arm for safety in case the threads strip, but don't apply pressure to the A-arm. Hold a big heavy weight (like an 8 or 10 pound sledge hammer) against once side of the lower knuckle eye. Use a smaller (4 or 5 pound) hammer to hit the other side of the eye. This should pop the joint loose. I had no success beating the joint apart, but include it here for completeness since more than one person mentioned it.
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