Rear Door lock

On my 1998 GMC Safari I have three rear doors (the liftgate, and the two left/right swinging doors). I can unlock them by pressing the rear hatch button or the button on the keyless transmitter, but I can't unlock them with the key. If I try to unlock it with the key, I insert the key into the bottom right swinging door (this is where the key lock cylinder is located) and the key goes fully inside but does not want to turn left or right no matter how hard I try. How can I fix this?

If the lock cylinder does not turn, there is likely something mechanically wrong with the lock, or the key is worn to the point where it doesn't operate the tumbler plates to the correct depths any more. If you have a new (spare/backup) key, try that along with some light lubricant in the lock cylinder to operate the lock.

If you listen carefully as the key is inserted, you should be able to hear the lock plate tumblers riding up and over the key cuts, and actually feel them as the key is inserted. Since this lock cylinder is likely used a lot less than the doors and ignition, it would likely have the least amount of wear, and may be "tight" enough to reject a worn key.