Jack the right front up so the wheel/tire can spin freely (put it on a jack stand so you can get under the truck). Unscrew the actuator and use a socket wrench handle or extension inside the actuator socket to push the actuator towards the diff. while spinning the tire with the other hand.
It should lock up in just a few degrees of tire rotation. If it doesn't, the slider is buggered, if it does, the actuator is probably bad. If it locks up and the noise is coming from inside the diff, just replace the whole thing.
I bought an entire front diff for mine for only $125, so it's not worth worrying about a whole lot.