Can't unlock passenger doors

[spideylookalike]

2006 Sienna LE - Using either interior lock switches or keyless entry buttons, I am unable to UNLOCK any of the passenger doors (side, rear, etc.). I am only able to lock or unlock the driver's door. LOCKING of all doors is no trouble using any method (switch or keyless). I checked the door locking fuse and it is fine.

This just "happened" last night. After reading many of the posts which didn't exactly replicate my problem, I suspect this could be a relay problem. That is, assuming, there is a dedicated relay for UNLOCK of Passenger-only doors. I'm eager to get a response with suggestions. If you agree this could be a relay issue, how do I check it or replace it?

Thanks!