The shifts' harshness IS vacuum controlled. The MORE vacuum to the
transmission's shift modulator the SOFTER the shift will be, Conversely,
at wide open throttle NO vacuum is supplied, by design, so WOT shifts
are quick and crisp.
You need to check that the vacuum line to the modulator is intact. If
so, then there's a leak somewhere, probably in a door lock.
There's no shortcut, that's what needs to be investigated.