2002 kia optima 2.7L transmission replacement

Which end is the problem?

The outer end, on a lot of cars you have to undo one of the ball joints to get enough distance to wiggle out the splined shaft.

On the inner end there usually is an internal C clip that can hold the axle in pretty tight. You need a crowbar and a little leverage. Or a punch and a 5-pound hammer. These are tough shafts that can take a lot of abuse, so don’t feel bashful about banging and prying on them.