When you shut off the engine, pull the key and get out of the van, the computer resets the drive cycle. When you start the van and start moving again the computer checks the length of time it takes for the trans to complete a shift. If it's longer than 65 milliseconds, it increases the pressure. It will do this 5 times (so I've been told) before it gives up and changes the pressure to maximum. This makes the pump whine and the shifts jarring. The computer will store the code P1811 each time this occurs.
I know you already found the thread that explains this, but I'll put a link to it for anyone else who might be having this problem.