Scott,
As Mitch said, ArcGIS gets finicky sometimes with joins... If Dan's suggestion doesn't work for you still, you can try saving as a flat file (.txt, .csv). You can do this right from Excel, and then specify a schema.ini text file driver companion (ArcGIS should read this - just make sure it's in the same folder as your flat file):
https://msdn.microsoft.com/en-us/library/ms709353(v=vs.85).aspx
Inside of the ini, you can specify your flat file, datatypes, etc, e.g.
[Zips.txt]
Col1=ZipCode Text Width 5
Col2=WindSpeed Integer Width 2
You shouldn't have to do this, but it does allow you to get down to the basics and eliminate funkiness.