I think manually placing the lever in D will prevent the TCC from engaging, which is the test I wanted. Since there was no change, perhaps the transmission is not the problem.
I have had driveline vibrations in the past on mine, and every time it was wheel or tire related. Loose wheel nuts? Bent rim? On 3 separate occasions I have had tires break their steel belt internally, and the tire flops all around. In addition wheels out of balance will cause shaking. Perhaps change the rear tires to the front and vice versa would be a quick and cheap test. You can also see if there is any corrosion on the mounting surfaces on the back of the rim and the hub, and on the wheel nuts. Aluminum alloy wheels are very prone to corrosion, more so than steel wheels.
Have you done any work on the van to address any of the trouble codes?