So you want to view all records that have a 'grp' of A, B or C if South Shield is selected, and all records that have a 'grp' of D or E if Northwest Boreal is selected?
In your <option> statement, change the value to "'A','B','C'" and "'D','E'". Then, in your query where you have the LIKE statement, substitute it for the following:
AND (Stations.grp IN(" . stripslashes($region) . "))
The LIKE statement is good is you're searching parts of strings, but I think in this instance that the IN statement works better.