I have no first hand experience with MVs yet, however I do have a lot of experience with lockers andin particular with the detroit. You should be able to order a brand new.locker for the rears through any good diff shop ( I can recommend Randy's Ring & Pinion). In light duty trucks the Detroit locker replaces the entire differential carrier. In the 2 1/2 application the locker replaces just the side and spider gears. I haven't seen the 5 ton setup, but it most likely follows the 2 1/2 ton setup.
With regards to it allowing "slip" if you coast through a corner...a Detroit doesn't slip. It is either on or off. When you coast around a corner the dog gears disengage completely.
ARB has also done a limited run of air lockers for the 2 1/2 ton Rockwells from time to time. I don't know if they have ever done the same for the 5ton, but may be worth a phone call..would be nice to have at least for the front.