Passenger door lock issue

This is my first time posting a question for the forum, so I hope this is in the right location and that this question hasn't been answered before. I tried to search and did read several threads on similar issues, but hopefully I can get some help pointing me in the right direction.

I have a 1995 Infiniti G20t, and am having a problem with the factory keyless entry system, specifically the right front (passenger) door lock.

The door will only lock when I use the key and manually turn it. The door will not lock any other way, including when I 1) Press the lock button on the remote, 2) Throw the lock switch on the inside of the door, 3) Hit the lock button in the car that should lock all the doors, or 4) Turn the key twice in the driver's door to lock the car up.

In each of these ways, the lock switch on the inside of the door moves, so the door looks like it is locked, but when I check it from the outside, it is not locked. Also, when I move the lock switch back and forth quickly, there is a little bit of mechanical movement and the door makes a little bit of a clicking/whining sound, sort of like there are teeth that are being moved by a motor but are not engaging fully.

The door will unlock as normal, using any method I can think of.

From what I have read, this sounds like an actuator problem. But my mechanic wanted to charge me $45 to diagnose the issue, and said it could be a number of things other than the actuator. Is there anything I can be doing to further diagnose this issue? If it is the actuator, I am interested in buying one and attempting the install myself, if it is not too difficult. Thanks in advance for your help!