I'm not sure if I understand the requirement fully, but is it as simple as change the position 5 to 1?
SELECTION-SCREEN BEGIN OF BLOCK b3 .
SELECTION-SCREEN COMMENT 1(15) text-004 FOR FIELD p_depend .
PARAMETERS : p_depend RADIOBUTTON GROUP cdsd DEFAULT 'X' MODIF ID dep .
SELECTION-SCREEN COMMENT 1(15) text-005 FOR FIELD p_nodep .
PARAMETERS : p_nodep RADIOBUTTON GROUP cdsd MODIF ID dep.
SELECTION-SCREEN END OF BLOCK b3.