i got a 05 f150 xlt it grinds sometimes when driving but if i hit the 4wd switch it engages and stops .then back to 2wd..not sure what problem is .any help with this would be great.

 

Water intrusion in the vacuum lines can corrode the IWE and is sticks or there is insufficient vacuum to hold the IWE disengaged and it grinds. Check the lines and solenoid on the firewall for leaks and water intrusion. Also check the fuse.