Beginning in 1998, selected Chrysler products began rolling off the assembly lines with a new advanced security system known as the Sentry Key Immobilizer System (SKIS). Most WJ Grand Cherokees are equipped with this feature. When you attempt to start a vehicle with this feature, the onboard computer sends out a RF signal that is read up by the electronic transponder chip embedded in the key. The transponder then returns a unique signal back to the vehicle's computer, giving it the ok for the vehicle to start and continue to run. This all happens in under a second, and is completely transparent to the vehicle driver. For additional security, two pre-programmed keys are needed in order to register additional keys into the system. In the event of the loss of all keys, special programming equipment is needed to register new keys into the system.
It is highly recommended that you get a spare third key made and programmed for your Jeep. It's a good idea to do this while you still have your two original keys. If you were to ever lose one of those keys, it's a lot more costlier to have a replacement made as you must go to the dealer or a specialty locksmith to have the programming done. With your own pair of original keys, you can easily program a new key yourself.
The Sentry Key Immobilizer Module (SKIM) can be programmed to recognize up to a total of eight Sentry Keys. There are two possible methods to program the SKIM to recognize a new or additional valid key, the Customer Learn Method and the dealer-performed Secured Access Method. The Customer Learn programming feature is available only on U.S. vehicles and is detailed below. Note that once a Sentry Key has been programmed as a valid key to a given vehicle, it cannot be programmed as a valid key for use on any other vehicle. Blank keys can be typically found from a few different vendors on the Ebay auction site for around $20 plus shipping. Any key shop or hardware store can cut the keys.