I worked on a 99 Windstar that's similar to my 92 Explorer, so I think my remarks will apply to you....
Yes, it's likely the lock actuator. On the Windstar, the rear hatch wouldn't unlock, so I had to get the liner off and manually pull the rods going to each latch to open the hatch. I removed the actuator and verified everything worked smoothly without it before replacing the actuator. I'd remove the liner and take a look inside. I think you'll find the same thing I did.