Ok I am having this problem and I have done all the above switch relays, new relays, even new rear lock actuators. Well I work for a dealer and this is what we found.
If the rear doors don't lock try this and hope you dont have the same issue as me.
With the Drivers door open lock and unlock your doors if the rear doors did not lock
open the fuse door cover on the dash by the drivers door. Place a finger on the relays and lock and unlock the doors, you should feel the relays work.
Now depending on how hard you had your finger on relay the door may or may not lock. if it locked now remove your finger and do it again.
If it didn't lock you will need a new fuse box.
This is whats wrong with mine and its gonna get $$$$$. So I leave the cover off and when I need to lock it I just hold the relay in and lock the doors.
Whenever the wrecking yard near me gets an 03 chevy sub, k1500 or AV im gonna get the box but this is my fix till then.