You can try pulling the motor/actuator off the diff (it's real easy)- leaving the wires plugged in - flip the diff lock switch - if the motor turns - you know the motor is fine. Mine was gummed up and wouldn't turn at all when I tried the above procedure.

With the motor off - there is a hole in the diff (where the motor arm goes in) you should see some teeth of a slide gear (sorry don't know the real names) that slides left and right - which is what locks and unlocks the diff. Use a scredriver and slide it to the left - that should lock the diff. If yuo can lock it mannual - then I'm not sure what the problem is.

John