i have a 95 blazer i got a month ago. when i got the 4 wheel drive worked great, i used it alot then about a week ago the front hubs quit locking. the transfer case shifts fine and the drive shaft turns. i already replace the 3 pronge vacuum switch and it didint work. i replaced the bad vacuum lines and put a vacuum gauge on, and it has a stong signal. i was told the vacuum acuator goes bad but there is no rips and still moves . so any clues would be great. im very good with cars, but mainly older ones.