Solo pon la propiedad dragmode=1 en los textbox y pon algo asi en los eventos:
Código vb:
Ver originalPrivate Sub Text1_DragDrop(Source As Control, X As Single, Y As Single)
Dim Texto As String
Texto = Text1.Text
Text1.Text = Source.Text
Source.Text = Texto
End Sub
Private Sub Text2_DragDrop(Source As Control, X As Single, Y As Single)
Dim Texto As String
Texto = Text2.Text
Text2.Text = Source.Text
Source.Text = Texto
End Sub