3-4 Shift Valve
WARNING: This page is about a different car, the 2008 Isuzu Ascender, 2008 GMC Envoy, and 2008 Chevrolet TrailBlazer. However, it is still accessible from the selected car via links, so may be relevant.
Spring force holds the valve in the downshifted position, blocking 3-4 clutch fluid in preparation for a 3-4 upshift.