swap mapping of left and right shift keys

I have an odd, highly specific problem when using my third-party USB keyboard with my MacBook Pro: option + left-shift + right-arrow (which as Mac programmers will know means "highlight the word to the right of the cursor") does not work. The reasons behind this are unclear, but I know it's not to do with user-level software preferences, nor directly to with a low-level hardware problem (see my original question on superuser for the full troubleshooting logic).

It does work if I use option + RIGHT-shift + right-arrow but this is a very clumsy hand position for moving quickly around my documents. So I'd like to try swapping the Mac's interpretation of the left and right shift keys. Is this possible, and if so, how?