I've just fitted factory-spec Centre Diff Lock to my non-CDL-equipped Disco II. The mechanism was bought from Ashcroft. It's about an hour and a half's job to fit, then about 15 minutes to modify the SLABS ECU wiring so the ECU doesn't disable ETC and ABS when you start the car up with diff lock engaged.
So for a couple of hundred quid, I now have the benefit of CDL, without messing up the ETC.
If anybody's interested in the details, I've posted the method up in the disco2.com forums.
David