I get a terrible misfire in cylinder 8...ONLY when it rains and within a week or so after and then it goes away and doesn't come back till it rains again. how in the world do I fix this

 

Try replacing #8 spark plug wire. Might be a crack in the jacket letting some moisture through. Since you are changing one, you might as well change them all.

 

Motor? Year?

 

Try running some water on your windshield with a garden hose to see if you have a water leak at the base of the windshield.
Our 2003 Exped had the firewall pad misaligned at the factory allowing water to run under the pad and drip onto #6 & #7.

 

doesn't have spark plug wires

 

1999 5.4

 

Most likley the coil. Do you have wheel well liner 100%. If not #8 will get hit with road water.

 

well well liner??

 

Wheel well splash guards. Sounds like the Potting in the coil is starting to crack. Replace the coil.

 

How many miles on plugs and COPs - coils/boots/springs?
The coils will far outlast the boots/springs.
If you're pushing 100k on the the boots/springs it's probably a good time to replace them.
I replace plugs/boots/springs at 50k.
Still seems like you are getting rain water on #8 somehow.
Try swapping the #5 and #8 COPs to see if it is a COP issue.