I spent two hours to replace the transmission range sensor this afternoon. I can start my van now. Since I was still able to start the van with the old TRS just before the replacement, I can't be 100% sure that the problem is fixed. I will report back.
I also found a useful YouTube clip for this replacement job.
Change your safety neutral switch on a Ford Windstar 2000
http://www.youtube.com/watch?v=3inAO9gORh0