3-4 Shift Valve
WARNING: This page is about a different car, the 2008 Isuzu i-370, 2008 Isuzu i-290, 2008 GMC Canyon, and 2008 Chevrolet Colorado. 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.