1998 Ford F-150 4X4 will not engage

The four wheel drive will not engage. The selctor shoes that it is in 4wd but it is not. I also hear a clunk when I shift it into 4wd low. If you pull the vacuum off at the actuator on the front diff. Or pull the line at the switch on the firewall for the four whell drive it will engage. I tried replacing the switch but it did the same thing. I did not try to replace the 2wd switch only the 4wd one. These switch have 2 vacuum ports and and electrical conector. There is power to the 2wd switch all the time and when you turn the 4wd selctor to 4wd there is power to it. I have vacuum to both switchs all the time and so there is vacuum on both sides of the actuator on the front differential all the time.