My 2002 key fob has no issues opening the trunk, but locking/unlocking is weird.

Pressing the lock button does nothing to any of the four doors. Pressing the unlock will only unlock the back two doors, and not the front.

What do I need to replace to get the doors locking/unlocking like normal?

 

Check out the door lock actuator tutorial thread in the FAQ section, see post # 272 for a good writeup on accessing the actuator and the motor for it.