I recently purchased a 2005 Jeep Grand Cherokee (finally had to retire the 97 Grand Cherokee). However, I was given 1 key for it. The buttons on the key work to lock/unlock and the key starts the Jeep BUT when I manually put the key in the door to unlock, it doesn't turn the lock.
There isn't another lock on the Jeep so I am limited on troubleshooting this. Either the lock in the door is bad or the key is bad. I have a hard time believing the key is bad though because it starts the Jeep without a problem.
My TJ did that this winter, thought key cylinder was frozen. Thawed it out in a heated garage and still had the problem until I squirted WD-40 into the key hole and worked the key back and forth a little. Has been fine since and works smoother than I ever remember.
I had a key cut for my JK, and the ;ock smith cut it too deep. It started the Jeep fine, but won't open the doors. It could be the key. Just take it to a lockmith, and have a new key made from code (Use your VIN). If the new key still doesn't open the door, the you know the lock is bad. You really should have 2 keys anyways.
Like Vin said, good cheap way to diagnose the problem. Used to know a locksmith and watched him replace tumblers once and it looked pretty simple, probably not that expensive either. Tell the Locksmith the problem and he may be able to cut to the chase and fix it right away. Dont think you need to visit a dealer for this.