How Do I Get My New Key to Work Without My Old Key?

I stopped at the gas station today, and on getting back in my car, my key would no longer turn so I couldn't start the
vehicle. I tried everything for the next hour, to no avail. That key would not turn. I ran to the nearest auto parts store.
They sold me a new ignition cylinder and it included 2 new keys. I ran back to the station and installed it (T20 star
wrench required if you're trying this).  Hooray for me!  Problem solved!  Uh.. nope. I start it and 3 seconds later it dies
with a flashing 'SECURITY' on the dash. I soon realize there's some built in security feature with the key and so to get
my Durango to start, I now have to hold the original key right up next to the ignition slot while turning the new key to
start the car. After it starts, it's fine. I can put the old key down and drive.  Obviously, this ain't ideal but Google has not
helped me on the easiest way to solve this dilemma. Is this a "must buy a $100 key at the Dealer" kind of problem (in
which case I'll probably just duct tape the old key to the steering column) or can I reset the ECU (a term found from
Googling) to accept the new key only?  
Any help is greatly appreciated!  Thanks.