Maybe the problem is that the drives are mapped persistent and "Use" has problems with them. Unmap them first.
You do not have W9x, so there should be no problem using Z: to map to.
If I read through Jooel his code, he wil map the dives more consequent. So I think you have to add two more drive letters to your list.
Code:

Dim $strShareRoot, $IntNoUnits, $arrUnit, $arrShare, $arrDrive, $intNextDrive, $intUnit

$strShareRoot = "\\Server\"
$arrUnit = "_Prod", "_HR", "_AP", "_APSecr", "_FA", "Helpdesk", "_PZ", "_Dir"
$arrShare = "Prod$", "HR$", "AP$", "AP$", "Fin$", "IT$", "PZ$", "Dir$"
$arrDrive = "S:", "T:", "U:", "V:", "W:", "X:", "Y:", "Z:"
$FriendlyName = "S share", "T Share", "U share", "V Share", "W Share", "X Share", "Y Share", "Z Share"

For $intUnit = 0 To Ubound($arrUnit)
   If InGroup($arrUnit[$intUnit])
        Use $arrDrive[$intUnit] /DELETE /PERSISTENT
        Use $arrDrive[$intUnit] $strShareRoot + $arrShare[$intUnit]
        label($arrDrive[$intUnit],$FriendlyName[$intUnit])
   EndIf
Next

I think Label() is an UDF written by Jooel, you will find it in the UDF section.