1992 plymouth grand voyager with troubles

my problem is that my van will not shift in to the overdrive gear. It will go thru all the other gears. When it gets to the point to shift to its
last gear it wont. I have checked all the vacuum lines. I have also check very thing i can think of and i still can find it. It does this off and on. One day it will act normal and the next it will got right back. It you have ideas please email me
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