Trying to find duplicates in columns on two different spreadsheets, and
highlight them as yellow.

One file is "LeadShare" and the reference area is F2:F27

The other is "Install Customer List Pac NW- Diaz" and the reference area
is G4:G745

I want the values on the Install Customer list to show as yellow fill if
they are already stated on the first (Leadshare) list.

Here's one way with Excel 2003 without using a helper column.

First define a name "LS":
Insert Name Define
Then in the input field just below "Names in workbook" put this name:
LS
and for "Refers to" put
=LeadShare!$F$2:$F$27
and click "Add"

Then select G4 in the customer list and use
Format Conditional Formatting Formula Is
=COUNTIF(LS,G4)0
and choose the "Format" desired.

Then copy the cell format of G4 to the rest of the column.

Hope this helps.