Van power steering will go out for a few seconds at a time especially when it rains. There's also a squealing under the hood when this is going on. Any advice?

SOURCE: Serpentine belt keeps slipping off when it rains.

I have had the same trouble with my 2002 zx2. I have had to reinstall or replace the serpentine belt 4 or 5 times in less than a year. I have replaced the idler bearing and then the water pump went out which took out the belt as well. The belt still came off after replacing those items when I hit another puddle of water. I found that proper installation of the plastic engine splash guards which bolt to the bottom around the motor play an important roll in keeping out road water. Since the pulleys are accessible with the right front tire removed, I focused on securing the plastics tightly around the wheelwell framework. This has helped extremely in keeping water away from the pulleys. I haven't had any more troubles in the last 7 or 8 months. I'm still leary when driving in the rain as the routing of the serpentine belt on this car is the worst I've ever encountered.
Bottom line, check/replace idler bearing, tensioner if needed and make sure all plastic splash guards are there and properly screwed to the frame and each other.
Hope this helps.

SOURCE: loss of power going up hill

fuel pump is bad. they are under extented warranty from ford. take to dealer and "insist" they change it. there is service bulletin on this, but dealers often play "dumb".

SOURCE: Rain sometimes causes electrical problems

Find the source of the leak and fix that. Once water gets into the fuse box it will short circuit the componets such as fuses and cause all kinds of problems.

SOURCE: 95 ford windstar dies when i turn the steering wheel hard

windstar has a pressure switch on the pressuer line to the rack and pinion, squeeze the plug and pull up to disconect it and see if it happens again. you might have a short in one of these wires or the switch ... to reach the switch it is mounted on top of the power steering rack so reach down behind the motor on the driver side or crawl under and reach up on driver side

SOURCE: power steering failure/96 ford explorer

My sincere apologies for the delayed response.
I am personally going back to answer your unanswered questions.
I have only been a free contributor to this site for the past 4-months.

I do not know if you still need this information, but I am going ahead and answering it anyway.

CLICK on the following LINK. It has several Belt Diagrams, one which will match your specific engine and options (AC/No AC, Power Steering, .etc) that you will need.
To directly answer your question: YES the Power Steering Pump is part of the Serpentine Belt Route.
Let me know if this helped...
Again my apologies..

http://www.2carpros.com/car_repair_information/year/1996/make/ford/model/explorer/1996_ford_explorer_drive_belt_routing_diagram.htm