I have a 2004 ford f150 fx4 flairside with 150,000 miles. I notice that sometimes when i change it from 2 wd to 4 wd hi or low, i can hear a grinding noise coming from the front right wheel. I think its just not switching properly, but im not sure. Also i hear a loud knock from time to time ( also happens when i change from 2 wd to 4 wd ) Anybody what could be causing this and if so, how much to fix? Thanks!!!

 

Sounds like you hubs are not engaging. It can be either a mechanical problem at the wheel or a vacuum issue. There is a vacuum 'switch' located on the firewall, that If it gets moisture in it, it wont work.