There's already a small factory skid in place, and I beat on my stock axle pretty hard and never had a problem. I think there's more vital areas to protect first.
If you ever have a problem with the vacuum actuator not engaging, leaving you stuck without 4wd, simply remove the vacuum actuator from the front axle, slide the axle collar to the locked position, and install the vacuum actuator upside down. It will stay engaged and give you 4wd until you reverse the procedure.