Power Locks

I have a 2001 Chevy Impala 3.4L with 126,000 miles on it. The power locks will lock, but it won't unlock, the only way to unlock the doors is manually. The only way to unlock the doors from the outside is to use the key. If you press the lock button it will lock the doors, but if you press the unlock button it locks the doors also, when you shift it into park it locks the doors, and when you use the keyless entry unlock button, it locks them again. The mechanics at a local dealership told me all 4 actuators are bad, but that doesn't sound right to me. I replaced the driver side actuator, but still had the same problem. Is there a main relay that controls all four locks that could be causing all of them not to unlock? Or even a bad ground connection?