I would think the easiest way of getting a DIFF Lock in a truck that doesn't have one would be to have someone build you a new custom rear end. I would think it would be cheaper and easier then going to the dealer and having them installing one for you since they would probably sell you on a bunch of other useless stuff.
ARB Selectable Air Lockers are the best aftermarket lockers out there, period, and they just so happen to have one for the Tacoma. You'd have to change out the whole rear end to get the factory locker because the housings are different. ARB is the best way to go Below is the link to ARB's site. The part number for the locker is RD89 and it fits all Toyota 8" ring gears.