One thing to check is to make sure the push pull Bowden type cable has not separated.

Mine did that one time and come to find out the cable had separated at the sheath on the end of the cable. Fixed up a mounting bracket with small clamps on it to hold the sheath from moving but still allowed the inner core to move and it has worked flawlessly every since.

Was just thinking about this when I serviced the truck and tried to get some pics. It's almost impossible to get a good picture because of it's location above the muffler and it's dark and the flash overpowers things.

They are not the best pics but hope they describe what I am talking about. 

In the picture with the arrows the red arrow is the vacuum shift selector/valve, the yellow arrow is the direction that the cable lays, the bracket I made has the double bends in it to elevate the cable to the correct height to line up with the attachment point. The blue arrows are small bolts that hold the little cable clamps on the top side of the bracket. I was unsure how many clamps would be needed so I drilled 2 rows of holes so that I could alternate as many clamps as was needed. Ended up only needing  3. Might be worth a check I know a couple other folks that had the same thing happen.

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