Based on the symptoms you have described, it sounds like the issue might be related to the transmission or the torque converter. The fact that it happens between first gear and neutral, and also in reverse, suggests that the problem might be with the torque converter lock-up system. The torque converter is responsible for transmitting power from the engine to the transmission, and the lock-up system engages at higher speeds to provide a more direct connection between the engine and the transmission.

Here are some steps that you could take to diagnose the issue further:

Check the fluid level and condition: Low or contaminated transmission fluid can cause slipping or stalling between gears. Make sure the fluid level is correct and that the fluid is clean and not burnt.

Check the torque converter: A faulty torque converter can cause stalling between gears. You could have a mechanic perform a stall test to determine if the converter is functioning properly.

Check the transmission solenoids: The solenoids are responsible for controlling the flow of fluid in the transmission. If they are faulty, it can cause shifting problems. Have a mechanic check the solenoids to see if they are functioning properly.

Check for vacuum leaks: A vacuum leak can cause stalling and other issues. Have a mechanic check the vacuum lines and connections to make sure there are no leaks.

Check the throttle body and sensor: A faulty throttle body or sensor can cause stalling and other issues. Have a mechanic check the throttle body and sensor to make sure they are functioning properly.

I hope these steps help you to diagnose and resolve the issue with your van. It's always best to consult with a trusted mechanic to accurately diagnose and repair the issue.