i found my issue and am posting the solution just in case someone else runs into the same problem:

The data type was the issue. The Parameter 'DateRangeType' (Text data type) only determines which date range to use, either Fiscal Year or Calendar Year, but the actual VALUE being passed is Int (the year itself). i had to use the an expression to accurately define it and now it works! (whew!)

.rdl file now reads:

<Parameter Name="DateRangeType">

<Value>=iif(Parameters!DateRangeType.Value="FiscalYear", "FiscalYear","CalendarYear")</Value