Well replacement is always the best option, but if not I would try this. Start with a piece of flat wood. i.e. 2x6 2x8 2x10 or a piece of 3/4 plywood. Place some 16 penny nails in the board so that the upper end of the shaft is held in place. Now place a few nails further down the shaft stopping at where the bend begins. Slowly work the shaft by pushing against the nails. As you work the bend out, you may find that adding more nails as you go along will be helpful. Now for sue this is probably not the best method, and it may not work if the shaft is to thick. If it is really stubborn ask around and see if anyone has a pipe bender for household plumbing. Clamp one side down and work the bender slowly down till you get it where you want it.