Try using a 4# hammer as a backup on one side of the arm at the splines a hit the opposite side with a 2# hammer. Just as you think you can't make another swing it will pop loose. Leave nut on loosely to protect threads.
Yes the column will drop out the bottom IF you have a way to raise the front of the truck about three feet or have a lift to raise the whole truck.
We use an air chisel/hammer with an available smoothing hammer attachment that we ground a concave end on it to stay centered on the pitman arm eye. Pops them loose in seconds.

Evan