You could have just a very small leak and with the plug in it just built up over time. The plug should remain out unless you are going through deep water.
It can affect the operation of the truck if any of the grease and oil gets on the clutch. I'd try to clean it out the best I could without pulling the trans.
I'd clean it out, leave the plug out, check all my fluid levels, and run it for awhile to see if you've got a leak. If you overfill the trans it will leak into the bellhousing. It could be that somebody overfilled the trans in the past and the oil didn't have anywhere to go because the plug was in place.