Removed old thermostat and used two stubs of flex hose to bypass it, then drained and filled pan with fresh fluid. Unhooked return line at cooler and started engine to flush out cooler circuit. Plenty of flow through coolers and pushed a few more quarts of fresh ATF into system in the process. Next I installed a Derale 13011 fluid control thermostat up front and rerouted the lines. Had to cut one of the hard lines, so used a flare tool to make a lip to prevent the hose from slipping off and double clamped it to be sure. Transmission cooling system works properly, and the truck shifts like new, so I can drive it with confidence again.
Considering adding an external transmission filter, especially with all the crud I saw coming out of the coolers when I flushed them. Probably will do one more fluid drain and fill, but maybe in a few weeks time.