Engine Mechanical problem
1983 GMC Van V8 Two Wheel Drive Automatic 120000 miles

I have a 1983 GMC Diesel van that sounds like it has a dead battery when you crank it. I took both batteries out and checked them. Both are fully charged and putting out their recommended CCA (750). I took out the starter and tested it. It runs like a sewing machine. I took both batteries and the starter to a local parts store to double-check. All passed their tests with flying colors. While the starter was off, I tried turning the flywheel to make sure the engine wasn't locked down. It turned. With the starter remounted, I ran a wire from the battery to the starter to see if the switch was messed up. The starter kicked in. I bypassed the cables using jumper cables to connect the two batteries, and still everything sounds the same. So I have two good batteries, a good starter, a good switch, and good cables. And it still sounds like it has a dead battery. Anything else you can think to try would be greatly appreciated.