Ok since you are sure it is not the selector in the van or the linkage itsself then here are a few more
1. make sure you have the proper level of tranny fluid
2. It may be a Faulty fluid pump if the tranny isnt getting enough pressure it isnt going to shift.
Does reverse work ?
if both of these check out then i would suggest unless you have experience with automatic transmissions it would be best to take it to a tranny shop. Auto trannys are quite complex inside.