TH 400's will shift WITHOUT vacuum around 5500 rpm. This can be due to a modulator valve or a leak in the vacuum line, ie: rubber hose connectors at intake or the modulator itself.

Possible there could be an intake fitting leak, would also check the vacuum advance hose from carb to distributor. This link can mimic a bad trans vacuum issue.(along with carb issues)

Have had vacuum connections slip off, crack or simply clog from rubber tubing deteriorating from the inside out.

Then again, as mentioned, it could be internal and require a closer look.

Interested to know what you find, and good luck!

BG