1999 4L60E overdrive problems

The trans started out having problems downshifting from OD when going up hill but now it has started dropping in and out of OD while driving on level roads.
Is there a vacuum sensor on the engine (1999 5.3 Vortec) that sends a signal to the trans? If so, where would it be located? If not, what could cause this problem? Thanks, Arnold

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