Hi:) Most transmissions don't use vacuum to shift with anymore it is done by Hydraulic pressure and electronic servos operated by the Computer and a transmission that shifts hard could have a warren valve in the valve body that is letting to much Tranny fluid thought to the accumulators and that would cause the tranny to shift hard. Take it in to the shop before it destroy's the clutches in the transmission.
You have none and as for a vacuum leak no, Check engine light would pick up lean bank codes. aWhat you need to do is have the vehicle on a level ground engine warm vehicle in park check the transmission fluid,A low transmission fluid does and will cause a hard upshift as you have to raise the RPM. You should also change the transmission fluid with the filter. Also check the color of the transmission fluid it should be Red if its blackish it is burnt and the transmission service will not help you.
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