Dodge Ram 4WD engagement problem

I have a persistent problem with the 4WD on a 1997 Dodge 2500 Diesel P/U. The vacuum actuator has been repalced and the 4WD works for awhile, but after a few miles, there is a banging sound and 4WD stops working. If the actuator is removed it is damaged and will not engage anymore. I have tried locking the spline lock into place, but this lasts for only a short time. Could there be something wrong in the Diff case?