I did this at TechShop Detroit.
Step 1: What You'll Need
Philips screw driver
Step 2: Extend the Rod Fully
Using a set of vise grips, clamp the end to prevent it from retracting back into the shaft.
Step 3: Hammer Time
Using a hammer and your bench vise, continue hitting the bend in the rod until it straightens out. It won't take much. Keep rotating it and examining it to make sure it's straight from every angle.
Step 4: Adjust the Tension
Loosen the screw to make the door close faster. Tighten it to make it close slower. Play with it until the desired tension is achieved.
Re-install back on to your door and enjoy!