Have you cleaned the mast and then sprayed every joint with WD-40? Also, if you can hear that 'click click' when the motor is trying to go down it means the nylon guide is off. Before you replace it, clean the mast and push it down, pull it up to make sure that there isn't grit causing it to bind. Then, once its good and clean and lubed up, turn the car on and let the antenna motor finish. Then, have someone turn the truck off and when the motor starts to move, simply push down on the antenna at the same speed as the motor would pull it down. I have done this many times on Hondas to save replacing the mast. If it doesn't go down i'd bet there is just grit along the joints.
And if this doesn't work, you're not out any $$$ and you've confirmed that you need a new antenna. So its win win