Is it a hydro or gear drive? The transmissions are a little different. If you look on cubcadet.com under service/parts, you can put in the model number and look at all the exploded view drawings. The differential lock is part of the transmission, so there's no telling what's wrong. There are lots of parts in there and it could be a number of things. Also, the differential lock on these tractors is really pretty weak. Just a pin pushes into a hole in the ring gear, and holds it in place. It's not surprising that yours is not working, as this is probably the weakest thing in the whole tranny.
Also, not sure how close you've looked at it, but tearing into it to repair it may be a daunting task. There's lots of O rings and seals In a tranny, and if they're not all cleaned, dried, replaced, you'll probably end up with a leak. If you tear into it, plan on replacing all of the gaskets, seals, and O-rings! Also, the transmission is likely the hydraulic reservoir for the tractor, so everything needs to be cleaned-spotless when you're working, or a piece of crud in the tranny could plug up a line in the hydro control unit and shut it down. Sorry, but there's a reason these are often only repaired by dealers. At least there, if you get it back and it's not working, or it's leaking, they will have to fix it.