I have a 1994 Ford E350 van that every so often will have the speedometer go from crusing speed to zero. Also it will rev-up as if it was in neutral then goes back into gear. It has a automatic w/overdrive. It seems that the only time this happens is when the roads are wet. Someone told me it has something to do with a speed sensor that's located on the rear axle. If anyone can help it would be appreciated

Replace the speed sensor first, see if that corrects the problem.

Since you say it revs up as if it's in neutral - have you checked the transmission fluid level?

Revving up while in gear sounds like slipping - sometimes due to low transmission fluid level

"only happens when the roads are wet" sounds like you're losing traction - but I assume you'd know if you were just spinning your wheels...

I'm wondering if the speed sensor is giving bad info to the computer, which makes it think the tranny is in a different gear and causing it to rev up randomly... just a thought.

Could be, just throwing some other things out there - and transmission fluid level should be the first thing to check - certainly the cheapest and easiest solution - if it is a solution!

I don't understand how the transmission could shift out of, and then back into gear.
Check the transmission fluid level and condition. Has it been a while since it was service and had the filter replaced? May be due.
Then inspect the transmission connectors for water intrusion. If water is allowed to sit inside the connectors, corrosion will result and intermittent loss of those circuits.

I've checked the fluid levels and they are OK. The speedometer goes hairy just before the van starts acting up. The other day we had a mild spell where the highway was a little wet from melting snow and the van acted up, now the highway is dry and everything is working just fine. The only time it will act up is whenever the highway is wet, the van seem to be okay when I drive it around town, but when I hit the highway it will be OK as long as the road it dry.
I would like to know where I should look on the van for the problem equipment so I can do something about the problem. I've been thinking of making a shield of some sort to protect the piece from the moisture that is getting to it