on my 97 f150 with manual 4x4 but an automatic transmission, my 4hi or 4lo wont engage sometimes i pull the lever hear some clunking but the front wheels wont hook up. no matter how many times i try. and other days it slips right into 4hi with out a problem. can anybody help me?

 

I am not sure about the manual mode, but I had the same problem and it was a air switch on the transaxle. The part was like 200 and the mechanic installed it for $75
Hope this helps

 

Do a search in this forum. There are about 5 or 6 things that could cause the problem.