4x4 wont engage(grinds when i try)
 

1997 4.6ltr stick on floor wont engage at all. when i put it in 4hi on the fly there is a grinding noise. about 4 months ago i would put it in 4hi and it would take a while to hear the switch click behind the dash and then 4x4 light would come on. ever since then it has gotten worse and now it doesnt work at all. checked all fuses and all is well. could it be the relay switch? or a vacume line? where do i start to troubleshoot?