I have a 2005 f-150 that does not wnt to stay in 4wd. I can be driving down the road and it slips in and out. If i put it in 4wd drive at a stand still and try taking off it will grind and clunk,but it will go in 4wd while im driving without clunking but then it slips in and out!Any ideas would be appreciated!

 

ck the vacuum lines that run from the front diff to the top of the engine bay. May have a vacuum leak somewhere.