Help with key handling in DataGrid

Hi to all!
I added to DataGrid FullRowSelect behavior (in a new derived class DataGridEx).
Now I want keys LEFT and RIGHT to be handled as UP and DOWN keys respectively, that is when I press LEFT key the current selection in FullRowSelect mode must be shifted 1 row upper like it happens when I press UP key.
I do it through overriding PreProcessMessage method in DataGridEx class and replacing pressed key value from LEFT to UP and RIGHT to DOWN.
Everything goes OK.
Then I want TAB key alone to act like DOWN and SHIFT+TAB combination to act like UP key.
With TAB key alone it's OK again.
But SHIFT + TAB combination acts like SHIFT+UP combination, because I've been only replacing TAB key while it is pressed and DataGrid knows that I'm holding SHIFT key.
How to make system think that I pressed UP key not SHIFT + TAB combination?