Unfortunately, certain folks are wasting time and resources steering you to a vacuum problem when their understanding of the system is not comprehensive.

Understand this clearly:

Once vacuum to the transmission has fallen to such low levels (2-5" Hg), the vacuum system cannot fix the flaring. Flaring occurs when you've got too much vacuum. So, chasing vacuum issues at this point is a complete waste of time.

You've got two possibilities:

1) Get under the vehicle and increase the modulator pressure with the adjustment handle. Three turns clockwise on the handle and then take it for a drive.

2) Purchase the superior shift kit and replace some springs inside the valve body. This is not for the faint of heart and requires pristine cleanliness.